Would a reduction in the number of owned cats outdoors in Canada and the US increase animal welfare?


  1. Pre­vi­ously pub­lished es­ti­mates of the num­ber of birds and mam­mals kil­led by owned cats are prob­a­bly too large by over one billion an­i­mals. We es­ti­mated a me­dian of 640 mil­lion an­i­mals kil­led in both the US and Canada, whereas it has been sug­gested that a me­dian of 1933 mil­lion an­i­mals are kil­led in the US alone.

  2. Our es­ti­mates sug­gest that in­ter­ven­tions aimed at re­duc­ing owned cat pre­da­tion will there­fore di­rectly af­fect a smaller num­ber of wild an­i­mals (~830 an­i­mals per $1000) than pre­vi­ously sug­gested (~5500 an­i­mals per $1000)

  3. A sin­gle-mes­sage ad­vo­cacy in­ter­ven­tion, such as pro­mot­ing col­lars with pre­da­tion de­ter­rents (e.g. bells, bright col­ors) on all adopted cats, could have mul­ti­ple benefits for the welfare of both owned cats and wild animals

  4. How­ever, cat pre­da­tion may be a net benefit for wild an­i­mal welfare given the prob­a­bil­ity that this mor­tal­ity is of­ten com­pen­satory, the large re­pro­duc­tive ca­pac­ity of small ro­dents, and the in­hu­mane meth­ods of ro­dent con­trol cur­rently in use

  5. The wide­spread use of ro­den­ti­cides emerges from this anal­y­sis a wild an­i­mal welfare is­sue in need of fur­ther re­search with re­spect to the num­bers of tar­get and non­tar­get an­i­mals nega­tively impacted


Numer­ous ar­ti­cles have fo­cussed on the es­ti­mated num­ber of birds, and to a lesser ex­tent mam­mals and other an­i­mals, kil­led by cats (e.g., Blancher 2013; Loss et al. 2013). Th­ese pub­li­ca­tions have had large im­pacts in both the sci­en­tific and pop­u­lar press. Cat pre­da­tion is pre­sented as ei­ther a con­cern for con­ser­va­tion, or an­i­mal welfare, or both, and ac­tions to re­duce the num­ber of out­door cats, rang­ing from ex­ter­mi­na­tion to con­fine­ment, have been urged.

Scien­tific and con­ser­va­tion efforts have fo­cused on the po­ten­tial im­pact of cats on bird pop­u­la­tions. How­ever, in spe­cific ju­ris­dic­tions, such as Aus­tralia, other en­dan­gered ver­te­brates species are im­por­tant con­ser­va­tion tar­gets. There is no ques­tion that where cats are found in ar­eas with­out an evolu­tion­ary his­tory of similar meso­preda­tors, their effects on na­tive fauna can be dev­as­tat­ing. The most ex­treme cases are those where cats have been in­tro­duced to is­land lo­ca­tions with ground-dwelling birds. How­ever, even in these ex­treme cases, there are some in­stances of the benefits of cat pre­da­tion, through the limi­ta­tion of pop­u­la­tions of other non­na­tive species such as mice, rats and rab­bits on na­tive species of con­ser­va­tion in­ter­est (e.g., Karl & Best 1982; Flux 1993).

From an an­i­mal welfare point of view, cats are preda­tors and do kill other an­i­mals. Given the large pre­da­tion es­ti­mates offered by some au­thors, it seems nat­u­ral to fo­cus on re­duc­ing these kills rates to in­crease an­i­mal welfare. For ex­am­ple, Rowe (2018) com­pares var­i­ous in­ter­ven­tions and con­cludes that per­ma­nent con­fine­ment of owned cats is the most cost-effec­tive means of re­duc­ing wild an­i­mal deaths due to cats. How­ever, cats are op­por­tunis­tic and there­fore tend to favour abun­dant or vuln­er­a­ble prey items (Fitzger­ald & Turner 2000). Over­abun­dant prey species also nega­tively im­pact other in­di­vi­d­u­als via ter­ri­to­ri­al­ity, ag­gres­sive be­havi­our, steal­ing food, or kil­ling young, or more in­di­rectly through com­pe­ti­tion for limited re­sources (Gure­vitch et al. 1992 Heske et al 1994, Schoener 1983). When re­sources are limit­ing, the strongest com­pe­ti­tion is ac­tu­ally be­tween mem­bers of the same species. There­fore, there is a very real pos­si­bil­ity that cats, by kil­ling in­di­vi­d­u­als in these pop­u­la­tions, re­duce both the to­tal pop­u­la­tion growth rate and the num­ber of in­di­vi­d­u­als that will die of re­source limi­ta­tion or ex­po­sure. Since there are no good data-driven es­ti­mates of un­owned cat pop­u­la­tions in Canada and the US (Ap­pendix—Unowned cats), and since there is a large effort pro­mot­ing owned cat con­fine­ment, we fo­cus on the es­ti­mated pre­da­tion rates of owned cats in or­der to de­ter­mine if in­ter­ven­tions aimed at this pop­u­la­tion would have a large benefi­cial im­pact on an­i­mal welfare.

Pre­vi­ous es­ti­mates of to­tal cat pre­da­tion on birds and mammals

Es­ti­mates of the mor­tal­ity caused by cats have clearly in­di­cated the vast ma­jor­ity of cat pre­da­tion is due to un­owned cats (by which we mean un­do­mes­ti­cated or feral cats, semi-do­mes­ti­cated cats, and aban­doned do­mes­ti­cated cats), if es­ti­mates of both pop­u­la­tion sizes and pre­da­tion rates are rea­son­able. For ex­am­ple, us­ing data from the US and EU Loss et al. (2013) es­ti­mate that over two billion birds and twelve billion mam­mals are kil­led an­nu­ally by cats in the US. About 69% of this pro­jected mor­tal­ity is at­tributed to un­owned cats, with the re­main­der as­signed to owned cats. Un­for­tu­nately, the pop­u­la­tion es­ti­mates of un­owned cats have very wide ranges and very lit­tle data-based sup­port (see Ap­pendix—Unowned cats). The lack of data re­gard­ing the un­owned cat pop­u­la­tion is ex­tremely wor­ry­ing in that it drives these large es­ti­mates of wildlife mor­tal­ity. More­over, we have the ex­am­ple from Aus­tralia where es­ti­mates of the un­owned cat pop­u­la­tion, for many years pro­claimed to be be­tween 10 and 20 mil­lion (Dick­man & Denny 2010), may ac­tu­ally be as small as 2 mil­lion ac­cord­ing to re­cent analy­ses (Legge et al. 2017). Be­cause of the lack of data re­gard­ing un­owned cats, and also be­cause there has been so much con­ser­va­tion effort di­rected to­ward pro­mot­ing per­ma­nent con­fine­ment of owned cats as a benefi­cial in­ter­ven­tion, we fo­cus on owned cats for the re­main­der of this re­port – al­though, in pass­ing, we note that some of the pro­posed and im­ple­mented in­ter­ven­tions as­so­ci­ated with un­owned cats (e.g., poi­son­ing and shoot­ing) are quite in­hu­mane and si­mul­ta­neously un­likely to achieve stated aims.

The data and dis­tri­bu­tions used to gen­er­ate the pre­da­tion es­ti­mates for owned cats in some cases seems to be poorly cho­sen. For ex­am­ple, Loss et al. (2013) used liter­a­ture data from the US and EU to es­ti­mate that the base of the range of owned cat pre­da­tion on birds was be­tween 4.2 and 18.3 birds per year (Table 1 Loss et al. 2013). They mod­el­led the dis­tri­bu­tion of this data as uniform (i.e., as­sum­ing that the prob­a­bil­ity of the pre­da­tion of 6 birds per an­num was the same as 16 birds). An ex­am­i­na­tion of the data the au­thors used (Sup­ple­men­tary Table S1) sug­gests that a uniform dis­tri­bu­tion is not a rea­son­able choice (this was in­de­pen­dently noted by Wolf 2016), and that a log­nor­mal dis­tri­bu­tion could be a bet­ter fit (see Fig 1).

Figure 1: Data from 11 pub­li­ca­tions for the US and EU used by Loss et al. (2013) to es­ti­mate the base pre­da­tion rate of owned cats on birds with su­per­im­posed log­nor­mal and uniform dis­tri­bu­tions fit us­ing max­i­mum like­li­hood.

Re­cently, Loss and coau­thors have re­sponded to similar cri­tiques of their choices with vit­ri­olic at­tacks pub­lished in sci­en­tific jour­nals (Loss et al. 2018). Here we join with other au­thors in urg­ing that sci­en­tific in­ves­ti­ga­tion be con­tinued when there are real un­cer­tain­ties (Lynn et al. 2019). Con­trary to the au­thor’s claims, the model and data choices can have large effects on es­ti­mates. Loss et al. (2013) re­port a me­dian an­nual bird pre­da­tion rate of 684 mil­lion. Us­ing their origi­nal pa­ram­e­ters we find very similar re­sults (e.g., me­dian of 685 mil­lion, with 95% of the simu­la­tions be­tween 258 and 1513 mil­lion). If we use a log­nor­mal dis­tri­bu­tion in­stead of the uniform dis­tri­bu­tion to de­scribe the pre­da­tion rate per cat, then the me­dian es­ti­mated num­ber of birds kil­led by owned cats is re­duced by nearly 300 mil­lion, al­though, of course, the in­ter­val con­tain­ing 95% of the simu­lated es­ti­mates is much larger (Fig 2). Un­der­stand­ing how differ­ent data and dis­tri­bu­tion choices af­fect both the es­ti­mated mag­ni­tude of pre­da­tion, and the sen­si­tivity of these es­ti­mates to differ­ent in­puts, is im­por­tant for rel­a­tive com­par­i­sons be­tween sug­gested wild an­i­mal welfare in­ter­ven­tions.

Our es­ti­mate of owned cat pre­da­tion on birds and mammals

As a first step to eval­u­at­ing the ques­tion of whether in­ter­ven­tions for owned cats in North Amer­ica would have a sig­nifi­cant im­pact on bird and an­i­mal welfare, we ex­am­ined the cat pop­u­la­tion and pre­da­tion data used by Loss et al. (2013) and Blancher (2013) (see Ap­pendix—Owned cats, Diet and num­ber of prey). We find that for the US there is ev­i­dence that the to­tal num­ber of owned out­door cats has been over­es­ti­mated, and that this has im­pli­ca­tions for the es­ti­mated num­ber of birds and mam­mals kil­led. We also find it likely that the pre­da­tion rates of owned cats have been over­es­ti­mated (see Ap­pendix—Diet and num­ber of prey).

After eval­u­at­ing the choices made by these au­thors, we then calcu­lated our own es­ti­mated an­nual mor­tal­ity rates for birds and mam­mals in the US and Canada due to owned cats (where kills = # owned cats*per­centage out­doors*per­centage hunters*bird or mam­mal pre­da­tion rate*prey re­turn rate). We note that af­ter vet­ting only three pa­ram­e­ters in the Loss et al. (2013) model – the es­ti­mated num­bers of owned cats, per­centage of out­door cats, and owned cat pre­da­tion rates – our es­ti­mated me­dian num­ber of an­nual bird kills is over half a billion in­di­vi­d­u­als lower (see Ap­pendix, Table 1, Fig 2. Code and data at https://​​github.com/​​kcud­ding/​​ReThinkCats/​​). This differ­ence is pri­mar­ily due to the elimi­na­tion of sev­eral ques­tion­able refer­ences for owned cat pre­da­tion rates (see Ap­pendix—Pre­da­tion stud­ies that we ex­cluded).

Figure 2: Es­ti­mated me­dian num­ber of birds kil­led (and 95% in­ter­val) per year by owned US cats from 10,000 Monte Carlo simu­la­tions us­ing ei­ther the origi­nal pa­ram­e­ters and dis­tri­bu­tions in Loss et al. (2013), the Loss model with cat pop­u­la­tion num­bers ad­justed for cur­rent data (adj cats), or cur­rent per­cent owned cats al­lowed out­doors (adj out), or both (adj out & cats), us­ing a log­nor­mal dis­tri­bu­tion fit to the pre­da­tion data (lnorm pred), mak­ing all three ad­just­ments (adj out, cats, lnorm), or us­ing our model pa­ram­e­ters (Table 1). See Ap­pendix for com­men­tary on data sources and pa­ram­e­ter val­ues and https://​​github.com/​​kcud­ding/​​ReThinkCats/​​ for code.

Us­ing our pa­ram­e­ter and dis­tri­bu­tion choices, we com­bined the US and Cana­dian cat pop­u­la­tion data to es­ti­mate the an­nual num­ber of deaths due to cat pre­da­tion (see https://​​github.com/​​kcud­ding/​​ReThinkCats/​​). We pre­dict a me­dian ~140 mil­lion bird kills per year for owned cats in the US and Canada with 95% of the simu­la­tion data in the in­ter­val be­tween 44 and 397 mil­lion. For mam­mals we pre­dict a me­dian of ~500 mil­lion kills per year; how­ever, the in­ter­val con­tain­ing 95% of the es­ti­mated val­ues spans or­ders of mag­ni­tude be­tween 128 and 1764 mil­lion an­i­mals. Th­ese es­ti­mates sug­gest that those pub­lished by Loss et al. (2013) and Blancher (2013) could over­es­ti­mate the num­ber of birds kil­led by owned cats by over 600 mil­lion, and the num­ber of mam­mals by 700 mil­lion. Clearly the num­ber of an­i­mals kil­led by owned cats is still large, but for birds es­pe­cially, these num­bers are much smaller than other es­ti­mated sources of mor­tal­ity. For ex­am­ple, Loss et al. (2015) es­ti­mate the num­ber of bird deaths due to build­ing strikes in the US at ~600 mil­lion and car strikes at ~200 mil­lion. In ad­di­tion, con­sider that about 9 billion chick­ens are raised in cap­tivity and kil­led each year in the US (FAO 2019). The num­ber of mam­mal kills is much larger, but has very large un­cer­tainty. This smaller es­ti­mate of an­i­mals kil­led by owned cats has im­pli­ca­tions for the cost-effec­tive­ness of in­ter­ven­tions such as per­ma­nent con­fine­ment.

Table 1: Pa­ram­e­ter val­ues used to gen­er­ate our es­ti­mate of an­nual num­bers of birds and mam­mals kil­led by out­door owned cats in Canada and the United States (see Ap­pendix).

Eval­u­a­tion of in­ter­ven­tions to re­duce owned cat predation

A pre­vi­ous es­ti­mate of the num­ber of an­i­mal lives saved per $1000 spent on adop­tion level ad­vo­cacy urg­ing new own­ers to per­ma­nently con­fine their cats in­doors con­cluded that a me­dian of 5500 wild an­i­mals would be saved (with 95% of the simu­la­tions in the range of 2400 to 22,000) (https://​​www.getguessti­mate.com/​​mod­els/​​11008). The au­thors used to­tal kill rates from Loss et al. (2013) to reach this con­clu­sion, but used a smaller range of the per­centage of out­door cats than we did. Us­ing our num­bers (Table 1, with con­stants for rep­tiles and am­phibi­ans from this model added, since we do not have ro­bust data for these), that in­cluded a larger range for the per­centage of out­door cats, and the to­tal es­ti­mated owned cat pop­u­la­tion for both US and Canada, we ar­rive at a me­dian of ~830 an­i­mal lives not sub­ject to cat pre­da­tion (or 0.83 an­i­mal lives af­fected per dol­lar), with 95% of the simu­la­tions in the range be­tween 330 and 2150. This calcu­la­tion in­cludes the as­sump­tion of ~33 cats, which would oth­er­wise have been al­lowed out­doors, be­ing kept in­doors per­ma­nently as a re­sult of the $1000 worth of ad­vo­cacy. There­fore, this in­ter­ven­tion may lie within the range of cost effec­tive­ness for cor­po­rate cam­paigns aimed at farmed an­i­mals (Šimčikas 2019). Of course, the com­par­i­son here is be­tween num­ber of farmed an­i­mal lives af­fected by welfare changes and wild an­i­mals not kil­led. In ad­di­tion, we must also con­sider the re­pro­duc­tive efforts of the prey species in or­der to fully un­der­stand the im­pact (see Welfare im­pacts be­low). Un­for­tu­nately, com­par­i­sons to in­ter­ven­tions aimed at un­owned cats are be­yond the scope of this ar­ti­cle, given the ex­tremely large un­cer­tainty sur­round­ing the size of this pop­u­la­tion.

Fol­low­ing the ap­proach of Loss et al. (2013) and Blancher (2013), we es­ti­mated the per­cent var­i­ance ex­plained by each model pa­ram­e­ter us­ing the sum of squares from a mul­ti­ple lin­ear re­gres­sion (Fig 3). The pre­da­tion rate was the most im­por­tant quan­tity, fol­lowed by the es­ti­mated num­ber of cats, the cor­rec­tion fac­tor for the prey re­turn rates, the pro­por­tion of out­door cats, and the pro­por­tion of cats that are hunters. Sen­si­tivity, as in­di­cated by the stan­dard­ized re­gres­sion co­effi­cients, fol­lows the same pat­tern. A 10% de­crease in the per­cent of out­door cats (i.e., chang­ing the uniform dis­tri­bu­tion range to 0.225 to 0.475) de­creases the me­dian es­ti­mated num­ber of bird kills by ~10 mil­lion and that of mam­mals by ~43 mil­lion. In com­par­i­son, a 10% de­crease in the pre­da­tion rate (chang­ing mean from 3.7 birds/​yr to 3.4, and 13.3 mam­mals/​yr to 12.0 mam­mals/​yr) de­creases the num­ber of bird and mam­mal kills by ~15 mil­lion and ~53 mil­lion re­spec­tively.

The pre­dicted num­ber of deaths due to owned cats was most sen­si­tive to pre­da­tion rate. There­fore, cam­paigns aimed at con­vinc­ing own­ers to use col­lars with pre­da­tion de­ter­rents (e.g., bells, bright colours) may re­sult in fewer an­i­mal deaths than in­ter­ven­tions aimed at con­fin­ing cats in­doors if the in­ter­ven­tions have equal costs and effi­cacy. In the UK, hunt­ing cats wear­ing a bell re­turned 34% fewer mam­mals and 41% fewer birds than those with a plain col­lar (Nel­son et al. 2005). Cats known to be hunters wear­ing the Birds­be­safe col­lar cover kil­led 19 times fewer birds than un­col­lared cats in the spring, and 3.4 times fewer birds in the fall in New York state and did re­duce mam­mal pre­da­tion in the fall, but not the spring (Wil­son et al. 2015),. How­ever, this product has lit­tle im­pact on mam­mal pre­da­tion in Aus­tralia (Hall et al 2015). While it is pos­si­ble that there are nega­tive im­pacts of quick-re­lease col­lars with pre­da­tion de­ter­rents on cats, we could find no re­ports in the liter­a­ture.

Figure 3: The per­cent var­i­ance in the num­ber of birds or mam­mals kil­led ex­plained by each vari­able in our model of cat pre­da­tion as given by a mul­ti­ple lin­ear re­gres­sion, where cat is the es­ti­mated num­ber of owned cats in the US and Canada, out is the per­cent of these cats al­lowed out­doors, cathunt is the per­cent of out­doors owned cats that hunt, re­turn is the cor­rec­tion fac­tor to calcu­late ac­tual kills from prey re­turn rates, and pred is the pre­da­tion rate per cat.

It is likely that the costs for an adop­tion level cam­paign aimed at col­lar­ing cats with pre­da­tion re­duc­tion de­vices would be similar to the costs es­ti­mated for in­creas­ing the num­ber of in­door-only cats (Rowe 20188). Such a cam­paign may even have greater effi­cacy. In the UK, where per­ma­nent con­fine­ment is less pop­u­lar and con­cern for wildlife lower than other ju­ris­dic­tions (Hall et al. 2016), col­lar-mounted pre­da­tion de­ter­rents were con­sid­ered the most ac­cept­able in­ter­ven­tion (Thomas et al. 2012). How­ever, a com­plete eval­u­a­tion would also re­quire an es­ti­mate of the per­centage of cats that cur­rently wear pre­da­tion de­ter­rent de­vices. Wil­son et al. (2015) sug­gest that most cats wear no col­lar at all, but do not have data on this point. If true, then ad­vo­cacy on this po­si­tion could have a large po­ten­tial im­pact. More­over, Lord et al. (2010) in­di­cate that ~73% cats who have not pre­vi­ously worn a col­lar will tol­er­ate them well. Fur­ther, the au­thors also urge the col­lar­ing of in­door cats as a safe­guard for iden­ti­fi­ca­tion and re­cov­ery in the case of es­cape, in ad­di­tion to the use of microchips, since in­for­ma­tion on col­lars can be read­ily ac­cessed. There­fore, a sin­gle-mes­sage ad­vo­cacy in­ter­ven­tion, such as pro­mot­ing the use of a bel­led col­lar on all adopted cats, could have mul­ti­ple benefits..

Welfare implications

Be­fore con­tem­plat­ing any in­ter­ven­tion, how­ever, it is worth ex­am­in­ing which in­di­vi­d­u­als are likely to be tar­geted by cats in or­der to un­der­stand the welfare im­pli­ca­tions of this pre­da­tion. Par­tic­u­larly for mam­malian prey, it is un­clear whether cat pre­da­tion is net nega­tive for an­i­mal welfare. In ad­di­tion, there is also the welfare of the cats them­selves to con­sider.


While Amer­i­can and Cana­dian vet­eri­nary as­so­ci­a­tions recom­mend in­door con­fine­ment as a way to im­prove welfare, cats that are con­fined may be sub­ject to phys­i­cal and men­tal health is­sues from the as­so­ci­ated in­ac­tivity, bore­dom or stress. For ex­am­ple, Slin­gler­land et al, (2007) found that the in­ci­dence of di­a­betes was sig­nifi­cantly as­so­ci­ated with phys­i­cal in­ac­tivity and con­fine­ment. This re­la­tion­ship be­tween in­door con­fine­ment and di­a­betes is sup­ported by other stud­ies (Öh­lund et al. 2017). The as­so­ci­a­tion is al­most cer­tainly due to a higher in­ci­dence of obe­sity in con­fined cats (e.g., Gib­son et al. 2018). In ad­di­tion to di­a­betes, obe­sity in­creases risks of hep­atic lipi­do­sis, lame­ness, oral cav­ity dis­ease, uri­nary tract dis­ease, der­ma­tolog­i­cal dis­ease, and neo­pla­sia (Tarkosova et al. 2016). Con­fine­ment it­self has also been as­so­ci­ated with higher in­ci­dences of feline urologic syn­drome, hy­per­thy­roidism and den­tal dis­ease (Buffing­ton 2002). In ad­di­tion, there is a higher in­ci­dence of be­havi­oural is­sues in con­fined cats (e.g., Amat et al. 2009), which is likely due to in­creased stress, in­suffi­cient stim­u­la­tion and lack of phys­i­cal ac­tivity (Bain & Stelow 2014). How­ever, cats with out­door ac­cess are more likely to be in­fected with par­a­sites or pathogens, some of them lethal (Chalkowski et al. 2019). They also en­gage in risky be­havi­ours out­doors such as cross­ing roads, en­ter­ing con­fined spaces and in­ter­act­ing with other an­i­mals that can re­sult in harm (Loyd et al. 2013a). For ex­am­ple, cat deaths un­der the age of 5 in Bri­tain are most of­ten due to road trauma (McDon­ald et al 2014). It is difficult to coun­ter­act some of the risks ex­pe­rienced by cats al­lowed out­doors. How­ever, while the nega­tive im­pacts of con­fine­ment can be coun­ter­acted by pro­vid­ing a stim­u­lat­ing en­vi­ron­ment and cre­at­ing op­por­tu­ni­ties for ex­er­cise, not all own­ers are will­ing or able to do so. We con­clude that the welfare im­pact on cats of con­fin­ing them in­doors seems un­cer­tain.

Prey species

The welfare im­pacts of cats on their prey is not nec­es­sar­ily nega­tive. Cats are op­por­tunis­tic preda­tors that will tar­get vuln­er­a­ble prey. For ex­am­ple, Liberg (1984) re­ports that dur­ing a harsh win­ter in Swe­den rab­bits were suffer­ing from star­va­tion and cats took a very large num­ber of rab­bits that he de­scribed as “dead, dy­ing and very weak”. Dur­ing that win­ter rab­bits made up 93% of all prey, and in­take was 6 times higher than in the pre­vi­ous years. Similarly, Baker et al. (2008) re­port that across species, birds kil­led by cats had sig­nifi­cantly lower mass, fat scores and pec­toral mus­cle mass scores than birds kil­led by col­li­sions with build­ings or au­to­mo­biles in Bris­tol UK, even though these birds did not differ in age, wing length or sea­son of death. There­fore, in some cases, cats may se­lec­tively tar­get mam­mals and birds in poor phys­i­cal con­di­tion (Dier­schke 2003; Møller & Er­rit­zoe 2000), and they are cer­tainly known to tar­get ju­ve­nile an­i­mals. Th­ese classes of in­di­vi­d­u­als may be ex­pected to have high nat­u­ral mor­tal­ity rates in the ab­sence of cats, such that cat pre­da­tion could wholly or par­tially com­pen­sate for nat­u­ral mor­tal­ity rather than be­ing ad­di­tive to it. In other words, in many cases, cats may prefer­en­tially kill in­di­vi­d­u­als that were likely to ex­pe­rience con­sid­er­able suffer­ing, and then death in the near term any­way.


As com­pared to small mam­mals (see be­low), the welfare im­pli­ca­tions of cat pre­da­tion on birds are par­tic­u­larly un­clear, and are prob­a­bly species-de­pen­dent. While some au­thors have sug­gested that cat pre­da­tion or even sub­lethal effects of preda­tory be­havi­our (Beck­er­mann et al. 2007) rep­re­sent an ad­di­tive mor­tal­ity fac­tor in main­land lo­ca­tions, there is lit­tle ev­i­dence to sup­port this claim. We know that birds kil­led by cats are more likely to have lower mass, lower fat re­serves, and poorer im­mune sys­tems than other birds (Dier­schke 2003; Møller & Er­rit­zoe 2000; Baker et al. (2008)). In ad­di­tion, cat pre­da­tion on birds is great­est in the spring, prob­a­bly re­flect­ing the kil­ling of ju­ve­nile in­di­vi­d­u­als (Baker et al. 2005; Lepczyk et al. 2003). How­ever, this is not ev­i­dence that these birds would have died a more pro­tracted or painful death in the ab­sence of cats. On the other hand, mere es­ti­mates of the mag­ni­tude of cat pre­da­tion or sub­lethal im­pacts tell us noth­ing about whether these an­i­mals would have sur­vived in the ab­sence of cats. For ex­am­ple, Mal­pass et al. (2018) found that while cats were 5x more likely to at­tack the nests of robins and car­di­nals in res­i­den­tial neigh­bour­hoods as com­pared to for­est parks of Ohio, there was no differ­ence in the nest sur­vival rates be­tween the two habitats. Even so, it seems plau­si­ble that cities and sub­ur­ban ar­eas with es­pe­cially high cat den­si­ties might ex­pose birds to ad­di­tive pre­da­tion. The ev­i­dence for this point is un­clear. For ex­am­ple, a meta-anal­y­sis of re­pro­duc­tive suc­cess in ur­ban en­vi­ron­ments found pat­terns of lower clutch size, lower nestling weight and lower pro­duc­tivity which the au­thors at­tributed to lower lev­els of nat­u­ral foods for nestlings (Cham­ber­lain et al. 2009). If true, this im­plies that nestlings and fledglings in ur­ban en­vi­ron­ments are more likely to starve than those in more nat­u­ral sur­round­ings. There is there­fore large un­cer­tainty re­gard­ing the welfare im­pacts of cat pre­da­tion on birds.


The small mam­malian prey favoured by cats, such as mice and voles, have short lifes­pans and enor­mous re­pro­duc­tive ca­pac­ity, which makes it quite likely that cat pre­da­tion rep­re­sents a net benefit to an­i­mal welfare for this group. To get some idea of the num­bers and as­ton­ish­ing ca­pac­ity for pop­u­la­tion growth, con­sider that Berry (1981) calcu­lated a sin­gle pair of house mice could the­o­ret­i­cally ex­pand to a pop­u­la­tion of 2,688 mice in only 6 months, as­sum­ing no mor­tal­ity and a lit­ter size of six young. Ges­ta­tion is only three weeks, and fe­males are sex­u­ally ma­ture at six weeks af­ter birth. How­ever, Berry and Jakob­son (1971) es­ti­mate that only about 5% of the pop­u­la­tion will sur­vive a harsh win­ter (al­though a mild win­ter can lead to an out­break pop­u­la­tion in the fol­low­ing sea­son). That is, most in­di­vi­d­u­als born are go­ing to die of ex­po­sure and star­va­tion within their first year. There­fore, if a cat kills a newly ma­ture fe­male mouse be­fore its first bout of re­pro­duc­tion, over the course of one sea­son it would po­ten­tially pre­vent the ad­di­tion of over 1000 ad­di­tional mice to the pop­u­la­tion (as­sum­ing a mor­tal­ity rate of 50% of the nestlings; Berry and Jakob­son (1971)), of which ~950 were go­ing to die over the course of the year (al­though note that this es­ti­mate was made in a nonur­ban en­vi­ron­ment; val­ues may differ where abun­dant hu­man re­sources are available). Re­con­sid­er­ing the costs of in­ter­ven­tion (see above), and as­sum­ing 70% of the prey were mam­mals similar to house mice, we might then con­clude that $1000 would po­ten­tially af­fect more than 29 000 mam­mals, where this figure in­cludes the origi­nal 581 mam­malian prey, and their po­ten­tial offspring, as­sum­ing a 50% sex ra­tio. There is con­sid­er­able un­cer­tainty re­gard­ing the cen­tral ten­dency and dis­tri­bu­tion of welfare lev­els of differ­ent groups of wild an­i­mals. In ad­di­tion, there is some un­cer­tainty about the rel­a­tive suffer­ing in­volved in differ­ent pos­si­ble deaths.

Whether or not you view a pre­da­tion death as a bet­ter or worse death than poi­son­ing, dis­ease, star­va­tion or ex­po­sure will also de­pend on how you scale the pain of these events vs. the length of the epi­sode. Cats kill by break­ing the neck of their vic­tims so that death is close to in­stan­ta­neous. In a con­trol­led study, Biben (1979) ob­served that adult mice are al­most always kil­led by cats with a sin­gle bite last­ing only a few sec­onds, while larger prey (young rats) re­quire over a minute un­til death. The in­ci­dence of effi­cient kills, with quick pur­suit and kill bite was cor­re­lated with the size of the prey and the hunger sta­tus of the cat. With larger prey and lower hunger lev­els, cats were more likely to en­gage in play be­havi­ours (Biben 1979). It is hy­poth­e­sized that this be­havi­our will ex­haust the prey so that in­jury to the cat is less likely. For the prey, this means that a pre­da­tion event could take longer, per­haps an hour or so, and more­over, the prey may not be kil­led, but in­stead merely fatally in­jured. For mice, star­va­tion with com­plete with­drawal of food can take rel­a­tively lit­tle time (2-3 days), but is prob­a­bly less likely than se­vere calorie re­stric­tion that takes longer to cause death. How­ever, there is ev­i­dence that the chronic pain of star­va­tion is dul­led, rel­a­tive to acute pain re­sponses, in starv­ing mice (Alhadeff et al. 2018). In their dis­cus­sion, Po­cock et al (2004) sug­gest that most com­men­sal mice in their study died by poi­son­ing and pre­da­tion, while wild liv­ing mice tended to die from ex­po­sure, star­va­tion and when pop­u­la­tion den­sity is high, dis­ease (Berry et al. 1973). There­fore, for ro­dents likely to be kil­led by owned cats, the most rele­vant dis­tinc­tion may be be­tween death by pre­da­tion vs death by poi­son­ing.

The high­est pop­u­la­tions of mice, like the high­est pop­u­la­tions of cats, are gen­er­ally found near hu­man habita­tion (Lau­rie 1946). How­ever, near hu­man habita­tion, agri­cul­ture and in­dus­try, ro­dent pop­u­la­tions are sub­ject to con­trol mea­sures such as trap­ping and poi­son­ing. Ro­den­ti­cides and kill-trap­ping meth­ods have as­so­ci­ated an­i­mal welfare (tar­get and non-tar­get) is­sues (Ma­son and Lit­tin 2003). In 2006, the EPA re­ported that 90% of of the $90 mil­lion spent by res­i­dents on ro­dent con­trol was dry bait, a cat­e­gory pri­mar­ily com­posed of an­ti­co­ag­u­lants (U.S. EPA 2006), with the re­main­ing 10% com­pris­ing snap traps, glue boards and similar items. For the pro­fes­sional pest con­trol mar­ket, 75% to 80% were ro­den­ti­cide prod­ucts, with 80% of these be­ing an­ti­co­ag­u­lents. The re­main­ing 20% to 25% in­cluded items such as traps and glue boards (U.S. EPA 2006). As well, the most com­mon self-re­ported form of ro­dent con­trol among farm­ers and pest con­trol agen­cies is the use of an­ti­co­ag­u­lant ro­den­ti­cides (Hind­march et al 2018, Mem­mott et al. 2017). Although some res­i­den­tial users in Cal­ifor­nia re­port they are slightly more likely to use snap traps than an­ti­co­ag­u­lants, while glue boards are the third most likely con­trol method for this group (e.g., Morzillo & Mer­tig 2011).

An­ti­co­ag­u­lant ro­den­ti­cides cause a lin­ger­ing death over days to weeks of se­vere hem­or­rhage, with mas­sive in­ter­nal bleed­ing and poor co­ag­u­la­tion and symp­toms as­so­ci­ated with blood loss such as ane­mia, pale mu­cous mem­branes, weak­ness, hy­pother­mia, and tachy­car­dia. More­over, these ro­den­ti­cides are known to be the source of sec­ondary poi­son­ing in re­lated preda­tor species (Erick­son & Ur­ban 2004), be­cause the poi­soned ro­dents take so long to die, and in their weak­ened state are an easy tar­get. Th­ese chem­i­cals are highly per­sis­tent, and ac­cu­mu­late in the tis­sues of species higher up in the food web (e.g., rap­tors, moun­tain li­ons, fish­ers), where they cause both lethal and sub­lethal effects. There is am­ple ev­i­dence for wide­spread food­web con­tam­i­na­tion driven by the use of ro­den­ti­cides (e.g., Gabriel et al.2018; Reg­n­ery et al. 2019; Sel­je­tun et al. 2019; Serieys et al. 2019). In ad­di­tion, non-tar­get co-oc­cur­ring ro­dents, birds, pets or chil­dren may in­gest the poi­soned baits. Un­for­tu­nately, new con­sumer mar­ket reg­u­la­tions on the more lethal sec­ond gen­er­a­tion chem­i­cals do not seem to be hav­ing a large im­pact on over­all use or non­tar­get wildlife con­tam­i­na­tion (Mur­ray 2017).

The use of ro­den­ti­cides is re­lated to hu­man oc­cu­pa­tion and land use, and there­fore over­laps those ar­eas where we could have a higher num­ber of out­door owned cats. A sur­vey of res­i­dents in Bak­ers­field, CA found higher use rates in sin­gle-fam­ily dwellings next to open spaces, as com­pared to multi-fam­ily dwellings in more ur­ban lo­ca­tions (Morzillo & Schwartz 2011), al­though is is pos­si­ble that con­trols are be­ing im­ple­mented by land­lords in denser hous­ing and were not de­tected by this sur­vey. No­geire et al (2015) used the Cal­ifor­nia agri­cul­tural database to clas­sify land-cover types with re­spect to an­ti­co­ag­u­lant ro­den­ti­cide use, as well as the cor­re­spond­ing mea­sure­ments of ex­po­sure of kit foxes in these ar­eas (Cypher et al 2014). Areas with con­fined an­i­mal agri­cul­ture, semi-agri­cul­tural ar­eas or low-den­sity res­i­den­tial ar­eas (e.g. Morzillo & Mer­tig 2011) were as­signed the high­est us­age rates. Ur­ban lands and or­chards were scored in­ter­me­di­ate use while nat­u­ral land cover, cropland or graz­ing lands were given a low use score. Out­door cat den­si­ties may be high­est in res­i­den­tial ar­eas and in­dus­trial ar­eas (Flock­hart et al. 2016), and could po­ten­tially be ex­ploited to help con­trol ro­dent pop­u­la­tions and re­duce the use the use of ro­den­ti­cides.

It is cer­tainly clear, how­ever, that out­door cats alone can­not fully con­trol pop­u­la­tion growth of of small ro­dents. For ex­am­ple, Liberg (1984) es­ti­mated the num­ber of in­di­vi­d­ual field voles and wood mice con­sumed by cats in an area of 4500 hectares in Swe­den over 2 years as ~31 000 voles and ~5000 mice, only about 20% of his es­ti­mated pro­duc­tion of new voles and mice of 171 000 and 20 000 in­di­vi­d­u­als re­spec­tively. How­ever, in suffi­cient num­bers they may ex­ert enough of an in­fluence to make lo­cal pop­u­la­tion in­creases less likely, and to re­duce the need for less hu­mane con­trol meth­ods. For ex­am­ple, some own­ers keep farm cats in or­der to con­trol ro­dent pop­u­la­tions (Crowley et al. 2019), or re­port that cats re­duce ro­dent prob­lems (Hind­march et al 2018). Aside from pre­da­tion, out­door cats may also re­duce the ro­dent pop­u­la­tions near hu­man in­ter­ests by cre­at­ing a “land­scape of fear” (e.g., Themb’al­ilahlwa et al. 2017). While some stud­ies in­di­cate that cats are not effec­tive preda­tors of larger ro­dents (e.g., Nor­way rats Glass et al. 2009), a com­bi­na­tion of low lev­els of pre­da­tion and the pheromones re­leased by the cats may be enough to re­pel most species. For ex­am­ple, sev­eral an­i­mal char­i­ties run “cats at work” pro­grams for un­owned cats that are not adopt­able as pets. Th­ese an­i­mals are given food, shelter and med­i­cal care in re­turn for pa­trol­ling ar­eas that are plagued by ro­dents, and seem to have a high suc­cess rate at re­duc­ing lo­cal pop­u­la­tion prob­lems (e.g., Wash­ing­ton Post 2016). Such changes in oc­cu­pa­tion could re­duce the use of ro­den­ti­cides since res­i­den­tial users of­ten re­port that it is the visi­bil­ity of ro­dents that leads them to em­ploy con­trol mea­sures (e.g., Hind­march et al 2018). There­fore, out­door owned cats may si­mul­ta­neously re­duce the need for in­hu­mane ro­dent con­trol near hu­man in­ter­ests, and re­duce fast pop­u­la­tion growth rates that lead to larger amounts of an­i­mal suffer­ing.


We find that the pre­vi­ously pub­lished es­ti­mates of bird and mam­mal kills due to owned out­door cats are prob­a­bly in­flated. As a re­sult, in­ter­ven­tions aimed at in­creas­ing wild an­i­mal welfare via meth­ods to re­duce this pre­da­tion prob­a­bly have a lower cost-effec­tive­ness than pre­vi­ously calcu­lated. The es­ti­mate of deaths due to owned cats was most sen­si­tive to pre­da­tion rate, sug­gest­ing in­ter­ven­tions that af­fect this pa­ram­e­ter di­rectly, such as ad­vo­cat­ing the use of cat col­lars with pre­da­tion re­duc­tion de­vices (e.g., bells), may have a larger im­pact than efforts aimed at keep­ing cats in­doors. More­over, use of pre­da­tion re­duc­tion de­vices may be more palat­able than per­ma­nent con­fine­ment to some own­ers. How­ever, given that the vast ma­jor­ity of pre­vi­ously es­ti­mated cat pre­da­tion (~70%) was at­tributed to un­owned cats, even a 100% effec­tive in­ter­ven­tion for owned cat pre­da­tion would ad­dress only a small pro­por­tion of wild an­i­mal deaths due to cats. Thus re­search aimed at pro­vid­ing data-based pop­u­la­tion es­ti­mates for un­owned cats is sorely needed. This is par­tic­u­larly true since the welfare of un­owned cats them­selves may also be quite low.

Fi­nally, we note that there is a rea­son­able pos­si­bil­ity that cat pre­da­tion is ac­tu­ally a net an­i­mal welfare benefit. Cats may tend to tar­get an­i­mals that have a high prob­a­bil­ity of a slower death due to poor con­di­tion or high ju­ve­nile mor­tal­ity. They may also re­duce the need for more in­hu­mane pest con­trol mea­sures in ar­eas of hu­man oc­cu­pa­tion. In fact, use of ro­den­ti­cides emerges from this anal­y­sis as an an­i­mal welfare is­sue of po­ten­tially large mag­ni­tude wor­thy of fur­ther in­ves­ti­ga­tion. We con­clude that it is un­clear whether re­duc­ing the num­ber of out­door owned cats would have a net benefit on an­i­mal welfare.


Cur­rent pop­u­la­tion estimates

The to­tal num­bers of cats in Canada and the US may be use­fully di­vided be­tween owned cats and un­owned cats (by which we mean un­do­mes­ti­cated or feral cats, semi-do­mes­ti­cated cats, and aban­doned do­mes­ti­cated cats). Of these pop­u­la­tions, only those cats that have ac­cess to the out­doors are con­sid­ered to have a sig­nifi­cant im­pact on ro­dents and birds for the pur­poses of this re­port.

Our re­view of the liter­a­ture makes it seem very likely that the es­ti­mated num­bers of out­door cats in the US, and pos­si­bly those in Canada, used in the con­ser­va­tion liter­a­ture are over­es­ti­mated. In a re­cent con­tri­bu­tion, Rowan et al. (2019) and Wolf (2016) similarly ar­gue that there are far fewer out­door owned cats than these pa­pers sug­gest. Pub­lished pa­pers that in­clude es­ti­mates of the num­ber of owned cats in the US nor­mally only cite two sources that are al­most cer­tainly in­flated. There seems to be only one na­tional sur­vey for Canada. The per­centage of owned cats that spend time out­doors has also been es­ti­mated on a rather ad hoc ba­sis in most pub­li­ca­tions, al­though there is sur­vey data. For un­owned cats, pre­vi­ously pub­lished es­ti­mates are pri­mar­ily based on ex­pert opinion rather than data, and there­fore have large mar­gins of er­ror.

Owned cats


Two sur­veys of pet pop­u­la­tions in the US are com­monly used to es­ti­mate the num­ber of owned cats. The Amer­i­can Ve­teri­nary Med­i­cal As­so­ci­a­tion (AVMA) con­ducts a sur­vey ev­ery 5 years us­ing rep­re­sen­ta­tive data (non­ran­dom) of ~50,000 house­holds, while the Amer­i­can Pet Prod­ucts As­so­ci­a­tion (APPA) also uses a non­ran­dom sam­ple and had ~22,000 re­spon­dents in 2017. Rowan et al. (2019) re­port that the AVMA sur­veys in­di­cate a vir­tu­ally un­changed rate of cat own­er­ship (around 30–30.5% of house­holds) from 1986 to 2011 while the APPA sur­veys show a steady in­crease in the rate of cat own­er­ship (from 30 to 37% over the pe­riod from 1988 to 2016).

Cur­rently both sur­veys use In­ter­net “opt-in” pan­els, which are much cheaper than ran­dom­ized mail, tele­phone or in-per­son meth­ods. The re­sponses are typ­i­cally weighted to match U.S. de­mo­graph­ics, but this may not be suffi­cient to pro­duce rep­re­sen­ta­tive data. Opt-in sur­veys fo­cused heav­ily on one topic, such as pet own­er­ship, are more likely to elicit re­sponses from pet own­ers. The Amer­i­can As­so­ci­a­tion for Public Opinion Re­search warned that re­searchers should avoid this method for es­ti­mat­ing pop­u­la­tions (AAPOR 2010). It is quite likely that APPA sur­vey, with its fo­cus on pet own­er­ship and smaller sam­ple size is a less re­li­able es­ti­mate. Matt Salois from AVAM in­di­cated to the Wash­ing­ton Post (Jan 31, 2019) that they speci­fi­cally tar­geted non-pet own­ing house­holds in their 2016 sur­vey, which had 41,000 re­spon­dents. To ex­trap­o­late to na­tional num­bers, the es­ti­mated per­centage of cat-own­ing house­holds are mul­ti­plied by the av­er­age num­ber of cats owned per house­hold. The APPA sur­vey re­ports a higher per­centage of cat own­er­ship which leads to an es­ti­mate of 94 mil­lion owned cats in 2016/​2017 as com­pared to the AVMA es­ti­mate of 58 mil­lion for the same pe­riod.

Ac­cord­ing to the Wash­ing­ton Post (2019), the Sim­mons Na­tional Con­sumer Study, which is an om­nibus sur­vey that oc­curs an­nu­ally and which has ~25,000 ran­domly se­lected re­spon­dents con­tacted by mail and phone, found 53 per­cent of house­holds owned pets in 2016, which yields an es­ti­mate of 54 mil­lion cats, or about the same as the AVMA for the same year. The Amer­i­can Hous­ing Sur­vey from the US Cen­sus Bureau is con­ducted in per­son from a ran­dom sam­ple and has a re­sponse rate of 81% from 30,000 house­holds. This sur­vey first asked about pet own­er­ship in 2013 and found that 48% had pets, and 49% 2017. Th­ese es­ti­mates are both much lower than APPA es­ti­mate of 68% in 2017 and 67% in 2019.

Rowan et al. (2019) spec­u­late that even the AVMA num­bers may be too high, and provide an ex­am­ple from Mas­sachusetts from 1996 to 2011 that com­pares sur­vey re­sults from the Mas­sachusetts SPCA and the state level es­ti­mates from AVMA. The AVMA cat pop­u­la­tion es­ti­mates are about 50% higher than those of the lo­cal agency. In ad­di­tion, there is con­sid­er­able vari­a­tion in cat own­er­ship within and be­tween be­tween states. The more ru­ral states (MT, ND, ME, WY, VT) tend to have higher rates of cat own­er­ship (~45%) while LA, MI, MD and IL had the low­est rates at around 26-28%, so that ex­trap­o­la­tion based on na­tion-wide av­er­ages of cat own­er­ship, as done by the APPA sur­vey, may be prob­le­matic. As a best guess us­ing the AVMA and Sim­mons sur­vey, the cur­rent num­ber of owned cats in the US is more likely to be in the range of 50-70 mil­lion than the 94 mil­lion re­ported by APPA. Tak­ing the mean of these three es­ti­mates, we can de­scribe the es­ti­mated US pop­u­la­tion of owned cats as a nor­mal dis­tri­bu­tion with mean of 67 mil­lion and stan­dard de­vi­a­tion of 23 mil­lion. Us­ing this es­ti­mate with the Loss et al. (2013) model gen­er­ates a me­dian an­nual bird pre­da­tion rate for owned cats that is ap­prox­i­mately 170 mil­lion lower than the es­ti­mate pub­lished by these au­thors..


A na­tion-wide sur­vey con­ducted by Kynetec (formerly Ip­sos), es­ti­mated the num­ber of owned cats in Canada at 8.3 mil­lion in 2018 (CAHI 2019). This num­ber is based on re­sponses of 3,026 pet-own­ing house­holds, where the per­centage of own­er­ship ex­ceeded the high­est es­ti­mates of US own­er­ship at 37%. Th­ese find­ings are con­sis­tent with pre­vi­ous sur­veys done by Kynetec on be­half of the Cana­dian An­i­mal Health In­sti­tute (CAHI) since 2004. In­deed the sur­vey re­sults are vir­tu­ally iden­ti­cal to the 2016 on­line sur­vey by the IPSO with 1,222 re­spon­dents. We could find no re­ported er­rors about these es­ti­mates but Blancher (2013) uses a value of 8.5 mil­lion +/​- 0.25M in his 2013 study.

US and Canada

Ac­cord­ingly, we fit a nor­mal dis­tri­bu­tion to the es­ti­mated val­ues of owned cats in the US and Canada, and com­bined these val­ues. The mean num­ber of US owned cats was given as ~67 mil­lion (mean of the two sur­veys es­ti­mat­ing 54 mil­lion and the APPA sur­vey es­ti­mat­ing 94), for a to­tal North Amer­i­can­pop­u­la­tion de­scribed as a nor­mal dis­tri­bu­tion with mean 76 mil­lion and stan­dard de­vi­a­tion of 23 mil­lion.

Unowned Cats


Es­ti­mates of the num­ber of un­owned cats in the US vary wildly. More­over, there is lit­tle em­piri­cal data sup­port­ing these es­ti­mates. This state of af­fairs is un­for­tu­nate be­cause es­ti­mates of the num­ber of birds kil­led by cats are re­ported to be quite sen­si­tive to the num­ber of un­owned cats (e.g., Blancher 2013). Unowned cat pop­u­la­tion size ex­plained the great­est vari­a­tion in prey mor­tal­ity es­ti­mates (42% for birds and 51% for mam­mals) in Loss et al. (2013), while at the same time 69% of this mor­tal­ity was at­tributed to un­owned cats. Those au­thors re­port in the sup­ple­men­tal ma­te­rial that of the data sources they used “the val­idity of ex­trap­o­lat­ing three den­sity val­ues to a na­tional-scale abun­dance es­ti­mate is ques­tion­able”.

Loss et al. (2013) note that there are no data-based es­ti­mates of the un­owned cat pop­u­la­tion in the US, but use a uniform dis­tri­bu­tion with min­i­mum of 30 mil­lion and max­i­mum of 80 mil­lion to define this pop­u­la­tion in their model. Re­ports of un­owned cat pop­u­la­tion size that are sup­ported by data range from 10 to 50 mil­lion, al­though es­ti­mates with­out cited data range up to 100 mil­lion (e.g., Dauphine & Cooper 2009). Like other au­thors, Loss et al. (2013) try to ex­trap­o­late from lo­cal den­sity es­ti­mates to ob­tain a US-wide es­ti­mate. For ex­am­ple, Levy et al. (2004) gives a value of 50 mil­lion un­owned cats vs. an owned cat pop­u­la­tion of 73 mil­lion at that time. Her es­ti­mate is based in part on the APPA es­ti­mate of owned cats, the per­centage of house­holds re­port­ing that they fed un­owned cats in four coun­ties from Florida, Cal­ifor­nia, and Mas­sachusetts (~10%) and the house­hold es­ti­mate of the num­ber of cats fed (~3.5). The au­thor sug­gests that un­owned cats are a min­i­mum of 36–46% of the pop­u­la­tion of owned cats, and offers a calcu­la­tion of 0.5 times the num­ber of house­holds to ar­rive at her es­ti­mate. How­ever, in Maine, a sur­vey con­ducted Katie Lis­nik of The HSUS in­di­cated that the num­ber of un­owned cats may be no more than 10% of the pet cat pop­u­la­tion (cited in Rowan 2018). Rowan (2018) sug­gests that it is likely that states with hard win­ters will have far fewer out­door cats than states like Florida. Mer­ritt Clif­ton es­ti­mates that the un­owned cat pop­u­la­tion ranges from around 6.5 mil­lion in win­ter to around 12.5 mil­lion in sum­mer (re­ported in Rowan 2018).

Loss et al. (2013) at­tempt to ex­trap­o­late by calcu­lat­ing an av­er­age den­sity of cats in a lo­cal study, and mul­ti­ply by the area of the US. How­ever, sci­en­tific re­ports from around the world in­di­cate that cat den­sity away from hu­man habita­tion tends to be low (less than 1–5 cats per square kilo­me­ter; Liberg et al. 2000) un­less there is some con­cen­trated food source to sup­port higher cat pop­u­la­tions. For ex­am­ple, Gen­ov­sei et al. (1995) re­port a sta­ble den­sity of 1.5 cats/​km2 in an iso­lated area of Italy. Hansen et al. (2018) es­ti­mated the un­owned cat den­sity in a New Zealand na­tional park ad­ja­cent to hu­man habita­tion us­ing cam­era traps at be­tween 1.39 and 2.58 cats/​km2. Wor­ld­wide, these es­ti­mates are also similar to den­sity es­ti­mates for un­owned cats with a food source of sparse nat­u­ral prey (Liberg et al. 2000). In ur­ban cen­ters, these num­bers are much higher.

Data-based stud­ies sup­port the claim that cats are most of­ten found in the im­me­di­ate vicinity of hu­man habita­tion even in ge­o­graph­i­cally re­stricted re­gions. For ex­am­ple, Pi­quet et al. (2019), in a com­plete sur­vey of La Gra­ciosa, found that most cats were found near and within an­thropic ar­eas (rep­re­sent­ing 1.63% of the to­tal islet area), whereas the rest of the islet showed a lower cat den­sity. Typ­i­cally, dense pop­u­la­tions of cats oc­cur in lo­ca­tions where re­sources (e.g. food and shelter) are more con­cen­trated (e.g. Na­toli 1985; Liberg et al. 2000).

Gloss­ing over the var­i­ance in the spa­tial dis­tri­bu­tion of un­owned cats can lead to over­es­ti­mates of both their den­sity and po­ten­tial im­pact. The es­ti­ma­tion of un­owned cat num­bers in Aus­tralia offers a pre­cau­tion­ary ex­am­ple. For some years, it was widely claimed that there were be­tween 10 and 20 mil­lion un­owned, feral cats on the con­ti­nent (Dick­man & Denny 2010). How­ever, this num­ber was an un­sup­ported es­ti­mate (Lynn 2015). When biol­o­gists fi­nally un­der­took a com­pre­hen­sive as­sess­ment of cat num­bers in Aus­tralia they de­ter­mined that there were be­tween 1.4 and 5.6 mil­lion cats in nat­u­ral en­vi­ron­ments (the higher num­ber be­ing pre­sent fol­low­ing the rainy sea­son) and a fur­ther 0.7 mil­lion out­door un­owned cats in highly mod­ified ur­ban and sub­ur­ban en­vi­ron­ments, where mean cat den­si­ties are “hy­per-vari­able” de­pend­ing on food sub­sidy (Legge et al. 2017).

At­tempt­ing to use the same meth­ods as Legge et al. (2017), Rowan et al. (2019) es­ti­mate that about one quar­ter of all un­owned cats in the USA (~8 mil­lion) oc­cur in nat­u­ral ar­eas in the USA ver­sus 24 mil­lion in the rel­a­tively small ar­eas of highly mod­ified ur­ban and sub­ur­ban en­vi­ron­ments, for a to­tal of ap­prox­i­mately 32 mil­lion un­owned cats. In other words, the vast ma­jor­ity of un­owned cats in the US are con­cen­trated in the same ur­ban and sub­ur­ban ar­eas where hu­mans have highly im­pacted the en­vi­ron­ment. There­fore, we con­cur with Loss et al. (2013), that the num­ber of un­owned cats in the US may lie some­where in the range of 30 mil­lion to 75 mil­lion, but sus­pect that it is more likely to be on the low side of this range, or pos­si­bly even lower (see be­low re­gard­ing data on un­owned cats in one lo­ca­tion in Canada).


In Canada, Blancher (2013) notes that there are no data-based es­ti­mates of the un­owned cat pop­u­la­tion, and does in­cor­po­rate spa­tial vari­abil­ity in his es­ti­mate. He uses me­dia re­ports from an­i­mal care work­ers com­pared to the size of the hu­man pop­u­la­tion in the same mu­ni­ci­pal­ity to get a range in num­ber of un­owned cats per 1000 peo­ple of 50 and 150. That ra­tio was ex­trap­o­lated to all other mu­ni­ci­pal­ities in south­ern Canada, where south­ern Canada was defined to ex­clude 78% of Canada’s area that is in arc­tic and north­ern for­est ar­eas to give a range of 1.4 to 4.2 mil­lion un­owned cats in south­ern Canada.

If we com­pare this es­ti­mate to a lo­cal data-based es­ti­mate, how­ever, there are large dis­crep­an­cies. Us­ing dis­tance sight­ing meth­ods, Hand (2019) es­ti­mated un­owned cats at about 13.3 (95% CI 9.7–18.1) cats per km2 in Wind­sor, On­tario, for an es­ti­mated pop­u­la­tion size of 1,858 un­owned cats (95% CI 1,361–2,537). Wind­sor has a hu­man pop­u­la­tion of ~330,000 which yields a ra­tio of 4-8 cats per 1000 peo­ple. This value is an or­der of mag­ni­tude lower than the es­ti­mate used by Blancher (2013). Ac­cord­ingly, in the ab­sence of any other data, the pos­si­ble range of un­owned cats in Canada should prob­a­bly in­clude a smaller lower bound of 150,000 (37 mil­lion hu­mans*4/​1000). While this seems very low, it is worth point­ing out that only 33,491 (29%) of cats re­ceived by shelter or­ga­ni­za­tions in 2016 did not have iden­ti­fi­ca­tion (Hu­maneCanada 2017), and could there­fore pos­si­bly be pre­vi­ously un­owned. Although, some cats in trap-neuter-re­lease pro­grams will also have a tat­too or chip iden­ti­fi­ca­tion.

Out­door cats

Out­door cats com­prise two pop­u­la­tions: owned cats that spend time out­doors, and un­owned cats. We as­sume that all un­owned cats spend all of their time out­side. For owned cats, na­tional sur­vey data sug­gests that only ~30% of cats have out­door ac­cess; how­ever, pub­li­ca­tions con­cerned with con­ser­va­tion im­pli­ca­tions of cat pre­da­tion tend to use higher val­ues.

Loss et al. (2013) use a uniform dis­tri­bu­tion with bounds at 40 and 70% to de­scribe the per­cent of owned cats with out­door ac­cess. They claim that this value is cen­tered on the dis­tri­bu­tion for the three na­tion-wide stud­ies they cite. Of the three cita­tions, how­ever, two re­fer to the same sur­vey (Loss et al. 2018). Of the two in­de­pen­dent data sources, both date from the late 1990s even though there has been a con­sis­tent up­ward trend in the num­ber of cats kept strictly in­doors. One of the cita­tions is for APPA sur­vey data from 19967 even though more re­cent the APPA sur­veys in­di­cate that the num­ber of pet cats out­doors has de­clined steadily over the past decade (Fig 3 Rowan et al. 2019), with less than 10% out­side at night, and about 30% al­lowed out­side dur­ing the day.

Other stud­ies sup­port a lower per­centage than used by these au­thors as well, es­pe­cially if one con­sid­ers the differ­ence be­tween day­time, night­time and un­su­per­vised ac­cess. One study of 256 US house­holds sur­veyed from 1993-2003 found that only 17% of owned cats had un­re­stricted out­door ac­cess, and 50% were kept in­doors all the time (cited in Bern­stein 2007). Similarly, in a sur­vey of 184 cat own­ers at a vet hos­pi­tal Clancy (2003) found 97.1% were kept in at night. Hall et al. (2016) in in­ter­na­tional sur­vey found that US cat own­ers had one of the higher rates of cat con­fine­ment with only 44% al­lowed out­doors. Wolf (2016) in an ex­am­i­na­tion of 18 sur­veys sug­gests the per­cent of owned cats with out­door ac­cess is closer to 20-50%. In Canada, Blancher (2013) used a va­ri­ety of re­search to es­ti­mate that 40 to 70% of Cana­dian house cats are al­lowed to free-roam out­doors. How­ever, the most re­cent Kynetec sur­vey (CAHI 2019) gives the per­centage of owned cats per­mit­ted out­side with­out su­per­vi­sion as 28%.

There are sig­nifi­cant differ­ences in the time of day ac­cess to the out­doors, which given hunt­ing pat­terns, has im­pli­ca­tions for pre­da­tion rates. Sur­vey data also sug­gest that about half of these cats are al­lowed out­doors for no more than two to four hours each day (Clancy et al. 2003; Kays and DeWan 2004; Lord 2008). Wolf (2016) notes that is im­por­tant be­cause hunt­ing be­havi­our is not con­tin­u­ous. Lloyd et al. (2013b) found that only 13% of 24 pet cats ex­hibited be­hav­iors in­dica­tive of hunt­ing within their first five hours out­doors. In a back-of-the-en­velope calcu­la­tion, Rowan (2010) sug­gested that cats with semi-re­stricted ac­cess gen­er­ates about 12-15% “full time cat equiv­a­lents” of the ac­tual hunt­ing cat pop­u­la­tion.

We used a per­cent of owned cats with out­door ac­cess of 25–50%, which en­com­passes the lower bound set by re­cent na­tional sur­veys, and an up­per bound as recom­mended by Wolf (2016). When used with the Loss et al. (2013) model, this range of owned cats with out­door ac­cess yields a me­dian num­ber of an­nual bird kills that is ~330 mil­lion lower than the value re­ported by those au­thors.

Diet and num­ber of prey killed


Cat pre­da­tion will vary sea­son­ally, from one ge­o­graphic area and land­scape type to an­other, and ul­ti­mately from one cat to an­other (Bar­ratt 1998). That said, beside the house­hold food, the main diet of owned cats con­tains small mam­mals (mainly ro­dents), small birds or in­sects. The main prey are gen­er­ally in­sects and small mam­mals, but birds or Euro­pean rab­bit (e.g., Liberg 1984) are also im­por­tant. Con­sump­tion of rep­tiles can also be im­por­tant where these species are com­mon (e.g., Loyd et al. 2013b).

Like any preda­tor, we ex­pect cat pre­da­tion to vary with prey den­sity (e.g., Hol­ling 1959), in­creas­ing as prey den­sity in­creases un­til a sat­u­ra­tion den­sity is reached. In par­tic­u­lar, cats are known to be highly op­por­tunis­tic (e.g., Liberg 1984), so that their pre­da­tion rates will re­flect both prey abun­dance and vuln­er­a­bil­ity. For ex­am­ple, Lie­berg re­ports that dur­ing a harsh win­ter in Swe­den cats took a very large num­ber of rab­bits that he de­scribed as dy­ing from star­va­tion. On is­lands, cats may take a large num­ber of ground-nest­ing seabirds be­cause these species are both vuln­er­a­ble and abun­dant. More­over, since pre­da­tion rates are also re­lated to prey den­sity, data from over 50 years ago (i.e., be­fore dra­matic re­duc­tions in wild an­i­mal pop­u­la­tions, and in­ci­den­tally, be­fore the wide-spread use of com­mer­cial cat food), are likely to over­es­ti­mate cur­rent pre­da­tion rates for owned cats.

Vuln­er­a­bil­ity will be greater where the lo­cal fauna do not share an evolu­tion­ary his­tory with cats. In fact, Ross et al. (2019) have be­gun a pro­gram to fa­mil­iarize na­tive fauna with cats as preda­tors in an at­tempt to miti­gate the large im­pact they may have on Aus­tralian prey species. There­fore, pre­da­tion rates on is­lands, or in ar­eas such as Aus­tralia are quite un­likely to be rep­re­sen­ta­tive of pre­da­tion rates in North Amer­ica. In North Amer­ica, there are sev­eral na­tive small felids, and also similar-sized na­tive meso­preda­tors.


Num­bers of prey items for owned cats and un­owned cats are ex­trap­o­lated us­ing var­i­ous meth­ods such as ex­am­i­na­tion of stom­ach con­tents, anal­y­sis of scats or owner re­ports. Owner re­ported re­turns are the most com­mon method for owned cats. How­ever, these es­ti­mates can vary quite widely, per­haps due to meth­ods (e.g., sur­veys based on owner rec­ol­lec­tion vs ex­am­i­na­tion of re­turned prey items, ge­o­graph­i­cal lo­ca­tion, cat de­mog­ra­phy, etc.). In an of­ten cited pa­per, Crooks & Soulé (1999) es­ti­mate the num­ber of cats pen­e­trat­ing nat­u­ral habitat frag­ments im­me­di­ately ad­ja­cent to hous­ing de­vel­op­ments in San Diego and their kill num­bers based on re­sponses of owner rec­ol­lec­tion. The au­thors in­di­cate that cat own­ers re­ported that each out­door cat that hunted re­turned on av­er­age 24 ro­dents, 15 birds and 17 lizards to the res­i­dence each year (or 56 prey items per cat per year). How­ever, this figure is de­cid­edly higher than similar stud­ies in differ­ent lo­ca­tions. An ex­am­i­na­tion of prey re­turns by 70 cats in an English village over one year yielded a to­tal es­ti­mate of 14 prey items per cat per year (Churcher & Law­ton 1987). The Mam­mal So­ciety in the United King­dom re­ported prey kil­led or cap­tured by 964 owned cats dur­ing a five-month pe­riod in 1997 (Woods et al. 2003). The re­port doc­u­mented more than 14,000 prey col­lected by cat own­ers from their an­i­mals, and also yielded a mean es­ti­mate of 14 items per owned cat per year. A study in Cape Town (Mor­ling 2014) also gives prey re­turns of 14 items per year. In a study in Switzer­land, Tschanz and coau­thors (2011) note that a large frac­tion of own­ers con­sid­er­ably over­es­ti­mated their cat’s pre­da­tion, in­di­cat­ing that sur­vey­ing pre­da­tion rates by means of a ret­ro­spec­tive ques­tion­naire alone is not suffi­cient, and pos­si­bly ex­plain­ing the dis­crep­ancy be­tween the Crooks & Soule (1999) es­ti­mate and other stud­ies that re­lied on a closer re­port­ing of prey re­turn items.

Owned cat pre­da­tion rates

There are two ma­jor stud­ies on the topic of owned cat pre­da­tion in North Amer­ica. Blancher (2013) es­ti­mated that owned cats re­turned 2.8 to 14 birds per year range, with a higher value for ru­ral cats than ur­ban cats. Loss et al. (2013) provide a range of 4.2 to 18.3 birds per cat per year based on US and Euro­pean re­gion stud­ies. (Note that both au­thors ad­just these num­bers up­wards, for pre­da­tion events not de­tected by own­ers). Loss et al. (2013) provide es­ti­mated num­bers for ro­dents as well. For mam­mal kills by owned cats, they give 8.7 to 21.8 items (Loss et al. 2013).

Loss et al. (2013, suppl) and Blancher (2013) provide the refer­ences used to in­form the choices they made for the range and dis­tri­bu­tion of pre­da­tion rates of owned and un­owned cats. We used these lists as a start­ing point for our dataset of owned cat pre­da­tion rates, ex­clud­ing all refer­ences that had been ex­cluded for var­i­ous rea­sons by ei­ther of the two sets of au­thors (e.g., small sam­ple size, sur­vey data that re­quired rec­ol­lec­tion of past events). Be­cause cats are op­por­tunis­tic preda­tors (e.g., Liberg 1984), like these au­thors, we ex­cluded data from is­lands, where food choices would have been limited com­pared to main­land lo­ca­tions. We also ex­cluded data from Aus­tralia and New Zealand, where prey may not have an evolu­tion­ary his­tory that in­cluded small preda­tors like cats. In gen­eral, prey with­out this his­tory are less able to es­cape pre­da­tion, al­though there are promis­ing ex­po­sure tech­niques to in­crease prey sur­vival rates (e.g., Ross et al. 2019).

We note in pass­ing that that there is a sig­nifi­cant nega­tive re­la­tion­ship be­tween year of data col­lec­tion and re­ported pre­da­tion rates. We re­mind the reader that pre­da­tion rates are de­ter­mined by prey den­sity, that wild bird and mam­mal pop­u­la­tions have de­clined dra­mat­i­cally in the past cen­tury, and com­mer­cial cat food was not in com­mon use un­til some years af­ter World War II. While we did not ex­clude data based on year of col­lec­tion, it seems un­likely that cur­rent pre­da­tion rates for owned cats are well de­scribed by data prior to 1950.

The raw data from these pa­pers used by Loss et al. (2013) or Blancher (2013) is not given in ei­ther pub­li­ca­tion. In­stead, these au­thors both give calcu­lated an­nual rates based on cor­rec­tions for sea­sonal vari­a­tion in pre­da­tion suc­cess. Loss et al. (2013) state that the ad­justed par­tial-year pre­da­tion es­ti­mates are calcu­lated us­ing the av­er­age pro­por­tion of prey taken in each month from 4 differ­ent stud­ies of bird pre­da­tion, 2 of which in­clude data for mam­mals (Churcher & Law­ton 1987; Bar­ratt 1997; Fiore & Sul­li­van un­pub; van Heezik et al. 2010). Th­ese pro­por­tions are not re­ported. Blancher (2013) used the same ap­proach for birds and a similar se­lec­tion of year round stud­ies. How­ever, this au­thor does re­port the pro­por­tions used in a figure (Fig 1 Blancher (2013)). As a re­sult, it can be difficult to de­ter­mine what data from a given study Loss et al. (2013) or Blancher (2013) ac­tu­ally used in their re­port­ing. We provide some com­men­tary be­low on refer­ences that we de­cided to ex­clude, but which were used by these au­thors in their calcu­la­tions.

We used a com­bi­na­tion of the pa­pers from Loss et al. (2013) and Blancher (2013) that we did not ex­clude, and a few newer pa­pers that we un­cov­ered to cre­ate a set of 7 es­ti­mates of owned cat pre­da­tion rates. We par­ti­tioned these rates be­tween bird and mam­mal kills to al­low com­par­i­son to the liter­a­ture. The calcu­lated mean an­nual pre­da­tion rates, un­ad­justed for sea­son of data col­lec­tion, were fit to log­nor­mal, nor­mal and uniform dis­tri­bu­tions to de­ter­mine the best pa­ram­e­ters for simu­la­tions. For birds, a log­nor­mal dis­tri­bu­tion was a good de­scrip­tor, but for mam­mals there were no clear best fits (Fig 4), so we se­lected a log­nor­mal dis­tri­bu­tion for each group, be­cause of the ad­van­tage over the nor­mal dis­tri­bu­tion of ex­clud­ing nega­tive val­ues, and the ad­van­tage over the uniform dis­tri­bu­tion of not sug­gest­ing even prob­a­bil­ity for all val­ues (which does not seem to be true for birds).

Pre­da­tion stud­ies used for owned cat pre­da­tion data in Loss et al. (2013) or Blancher (2013) that we excluded

As pre­vi­ously men­tioned, we ex­cluded all data that had been ex­cluded by ei­ther Loss et al. (2013) or Blancher (2013). We also ex­cluded data from Aus­tralia, New Zealand and is­lands. Below we give our rea­sons for ex­clud­ing sev­eral pa­pers in­cluded by these two sets of au­thors that do not fall un­der these crite­ria.

McMur­ray and Sperry 1941: The largest re­ported pre­da­tion rate for owned cats in Loss et al. (2013) at 33.18 birds/​year is de­rived from this pa­per on the ba­sis of 2 birds found in the stom­achs of 22 cats, some of which may have been pets. How­ever, Blancher (2013) used this pa­per as a data point for un­owned cats only at 48 birds per year. Our value is 31 birds per un­owned cat per year.

Figure 4: His­tograms of an­nual es­ti­mated pre­da­tion rates from liter­a­ture data, and log­nor­mal, nor­mal and uniform dis­tri­bu­tions to this data via max­i­mum like­li­hood. (Please see https://​​github.com/​​kcud­ding/​​ReThinkCats/​​ for refer­ences and raw data.)

The pa­per deals with stom­ach con­tents from three differ­ent groups: Wi­chita Moun­tains Wildlife Re­fuge (n=24), the Fort Sill Mili­tary Reser­va­tion (22 from res­i­den­tial sec­tions and 13 from non- res­i­den­tial ar­eas), and those from other lo­ca­tions in Ok­la­homa (25). Twelve of the 107 stom­achs were empty and 11 more con­tained only traces of food, so per­centage tab­u­la­tions were based on the analy­ses of 84 stom­achs.

The three sites are de­scribed as:

Wi­chita Moun­tains Wildlife Re­fuge (24 sam­ples): Sam­pling dates not in­di­cated. In­cludes un­owned cats as well as some an­i­mals de­scribed as house­hold pets aban­doned at camp grounds. Birds re­ported as “trace”. Mam­mals as 54% weight.

Fort Sill Mili­tary Reser­va­tion (22 +13): There are two sam­pling lo­ca­tions here: “res­i­den­tial” and “non-res­i­den­tial” sec­tions. Authors state that most cats from res­i­den­tial sec­tions were trapped dur­ing the fall of 1939. Trap­ping in both sites de­scribed as fall and win­ter from mid-Oct to mid-Jan. Authors state that cats from res­i­den­tial sec­tions “prob­a­bly in­clude many so-called ‘alley cats’ as well as a few house pets”. Two birds, an English spar­row and a mead­owlark, formed the bulk of the last meal of 2 cats from res­i­den­tial sec­tions and made an av­er­age of 6.5 per cent of the food for the lot (so they are re­port­ing % mass). Only 1 of the 13 cats from non­res­i­den­tial ar­eas cap­tured a bird, a robin.

Cats Taken in Other Sec­tions of Ok­la­homa (25 stom­achs): Other sam­ples came from 12 coun­ties col­lected through­out the year and prob­a­bly in­clude some do­mes­tic pets that were hunt­ing along road­sides at night as well as more un­owned in­di­vi­d­u­als. Birds com­posed 6 per cent of the food and oc­curred in 5 of the 25 stom­achs. Re­mains of 3 birds – a Brewer’s black­bird, a horned lark, and a mead­owlark – were found in one stom­ach col­lected in Fe­bru­ary.

As far as we can tell, Loss et al. (2013) used the data of 2 birds in 22 stom­achs from the Fort Sill Mili­tary reser­va­tion as “Owned”. Th­ese cats are de­scribed by the au­thors as in­clud­ing “a few house pets” and “alley cats”. The un­owned data Loss and coau­thors re­port is then the sum of Wi­chita Moun­tains Wildlife Re­fuge cats, and the cats trapped in the non­res­i­den­tial ar­eas of Fort Sill Mili­tary Reser­va­tion. Since the au­thors of the origi­nal pa­per do not know what per­centage of their cat sam­ples that were owned cats, and re­port sim­ply that the one sam­ple “prob­a­bly in­cluded a few house pets”, this does not seem a good clas­sifi­ca­tion. More­over, only in­clud­ing full stom­achs will over­es­ti­mate pre­da­tion suc­cess. We record data from this pa­per as 9 birds in 107 stom­achs, to give an over­all pre­da­tion rate of 0.084 birds/​day for (31 per year) for pri­mar­ily un­owned cats in ru­ral ar­eas in 1941. There­fore, we do not use this data as an es­ti­mate for owned cat pre­da­tion. We note that it is not pos­si­ble to get pre­da­tion rates for mam­mals from this pa­per given that only per­cent mass is re­ported, not per­cent oc­cur­rence.

Eber­hard (1954): This au­thor ex­am­ined 202 cat stom­achs and re­ported on 121 stom­achs that con­tained prey items. Cats were taken by gath­er­ing of mo­tor traf­fic kills, shoot­ing, and trap­ping from 22 coun­ties through­out the state, the ma­jor­ity hav­ing been col­lected from Cen­tre, Mont­gomery, and Bucks Coun­ties of Penn­syl­va­nia. Most spec­i­mens came from farm ar­eas, al­though a lesser num­ber was taken from small towns. Those cats which were re­cov­ered or cap­tured in fields were tagged “field,” and those an­i­mals which were thought to be pets or were taken in towns were tagged “non-field”. Data were recorded sep­a­rately for spring and sum­mer, vs. fall and win­ter.

Loss et al. (2013) have ap­par­ently clas­sified the “non-field” cats as owned and used the fre­quency of bird re­mains oc­cur­rence re­ported by Eber­hard (birds found in 4 out of 47 “non-field” cats stom­achs, and field cats as un­owned birds in 24 out of 107 stom­achs). Cats in­cluded in the owned cat­e­gory al­most cer­tainly con­tained many un­owned cats since un­owned cat pop­u­la­tion den­sity is highly cor­re­lated with hu­man pop­u­la­tion den­sity. In ad­di­tion, 48 stom­achs were ex­cluded from the study be­cause they were empty. If this study is to be used is would prob­a­bly be bet­ter to re­port as 28 stom­achs out of 202 bird re­mains (as­sumed pre­da­tion rate of 0.14 birds/​day or 51 birds per year) for ru­ral ar­eas (farms and small towns) as prob­a­bly un­owned cats in 1949-1951.

Loss et al. (2013) de­scribe their calcu­lated rate of 124 ro­dents per year (found in 16 out of 47 cat stom­achs) for non-field cats as ab­nor­mally high, and ex­clude this data for owned cats, but re­tain the value of 297 per year for un­owned cats. Given the clas­sifi­ca­tion in­for­ma­tion and the ex­cluded stom­achs, it seems bet­ter to re­port this data as mam­mal re­mains found in 103 out of 202 stom­achs (pre­da­tion rate of 0.51 mam­mals/​day or 186 mam­mals per year per cat) as prob­a­bly un­owned cats in ru­ral ar­eas from 1949 to 1951.

Liberg (1984): The au­thor re­ports the fre­quency of oc­cur­rence of bird or mam­mal re­mains in scat col­lected from 1974 to 1979 in a ru­ral Swedish village. Scat is at­tributed to un­owned or owned cats on the ba­sis of lo­ca­tion of col­lec­tion and in some cases vi­sual id. The au­thor re­ports that about half the bird bio­mass in scats was Gal­liformes (e.g., chick­ens and pheas­ants). As a re­sult we were un­clear as to how to dis­t­in­guish kills of do­mes­ti­cated farm chick­ens from kills of wild birds, and we have there­fore ex­cluded the bird data from this pa­per. Loss et al. (2013) do not in­clude the mam­mal data from this pa­per, and our calcu­la­tions sug­gest that the mam­mal pre­da­tion rate would fall into the cat­e­gory of “ab­nor­mally high” which Loss and coau­thors used as grounds for ex­clud­ing sev­eral other stud­ies. There­fore, we have ex­cluded the mam­mal data as well.

Nel­son et al. 2005: The pa­per re­ports on prey re­turns for cats known to be hunters wear­ing ei­ther reg­u­lar col­lars or col­lars fit­ted with var­i­ous sound de­vices to de­crease pre­da­tion. Vol­un­teers were re­cruited from across the UK (mainly England) through ad­ver­tise­ments in var­i­ous cat and wildlife mag­a­z­ines, and a web­site. Cat own­ers were se­lected to take part in the pro­ject if they re­ported that their cat(s) kil­led at least one prey item a week and did not cur­rently wear a col­lar.

Of the origi­nal sam­ple of 150 origi­nal vol­un­teers, 16 dropped out of the study, while 62 failed to re­turn any prey. Since the study aimed at eval­u­at­ing the effi­cacy of de­ter­rent de­vices on ac­tive hunters, and since the au­thors could not dis­t­in­guish be­tween failure to com­plete the sur­vey and pre­da­tion failure, they ex­cluded these 62 sam­ples. As a re­sult, they also con­clude: “The study was not de­signed us­ing a ran­dom sam­ple of cats; in­deed cats were se­lected for their ten­dency to hunt. In view of this, it would be com­pletely in­ap­pro­pri­ate to es­ti­mate na­tional kill rates by ex­trap­o­lat­ing from the data re­ported here.” There­fore, we have ex­cluded this data.

Cole­man and Tem­ple (1996): This refer­ence is not a peer-re­viewed sci­en­tific pub­li­ca­tion and cites un­known data (i.e., there is no cita­tion list or even names and dates for data cited as “four other stud­ies”). Fur­ther, there are no pre­da­tion data nor meth­ods de­scribed. Other au­thors have cri­tiqued the use of this refer­ence, and have con­cluded that the other other data referred to may be from Er­ring­ton (1936), Par­malee (1953), and Eber­hard (1954). This con­clu­sion was pos­si­bly made by refer­ring to an­other pa­per: Cole­man, J. S., & Tem­ple, S. A. (1995). How many birds do cats kill. Wildlife Con­trol Tech­nol­ogy, 44, which we can­not lo­cate. Wolf (2016) claims that the three men­tioned stud­ies were already in­cluded in the Loss et al. (2013) data set, and that the fourth (Mitchell and Beck 1992) was ex­cluded by Loss et al. (2013) due to its small sam­ple size. The 4-year pre­da­tion study for Wis­con­sin that the au­thors men­tion may be ei­ther a sur­vey on cat own­er­ship they con­ducted, or some un­pub­lished study. Loss and coau­thors (2013) use this pa­per for un­owned cat pre­da­tion rates while Blancher (2013) uses it for ru­ral owned cats. How­ever, as far as we can tell, this pa­per does not in­clude origi­nal data and has been ex­cluded.

Baker et al. (2005): Loss et al. (2013) use both this pa­per and Baker et al. (2008), how­ever, the 2008 pa­per, in part, re­ports on the 2005 data. As a re­sult, we have used only the 2005 data re­ported in Baker et al. (2008). Blancher (2013) only re­ports on the 2008 pa­per, and gives a com­par­a­tively low bird pre­da­tion rate (0.6 birds/​year, whereas we calcu­late 1.75 birds per year with­out sea­sonal ad­just­ment for both the Baker (2005) and the Baker (2008) stud­ies). We have ex­cluded this pa­per be­cause of pos­si­ble du­pli­ca­tion.

Per­centage of out­door owned cats that hunt

In all of the stud­ies of owned cat pre­da­tion, there are some num­ber for which pre­da­tion is not de­tected. This may be be­cause cats do not nec­es­sar­ily re­turn prey to own­ers, own­ers do not re­spond to re­quests for in­for­ma­tion, the sam­pling in­ter­val is too short to de­tect in­fre­quent pre­da­tion events, or be­cause the cats do not suc­cess­fully hunt. For ex­am­ple, Baker et al. (2008) re­port that only 40% of cats in the study re­turned any prey to their own­ers; how­ever, this ab­sence does not nec­es­sar­ily in­di­cate failure to hunt. Loyd and coau­thors (2013b) placed video cam­eras on 55 cats and ob­tained on av­er­age of 7-10 days of to­tal footage. Of the 55 cats, only 24 were ob­served to en­gage in “pre­da­tion be­havi­our” of stalk­ing or chas­ing prey, and only 16 (~30%) were ob­served to ac­tu­ally cap­ture prey.

In ad­di­tion, stud­ies have in­di­cated that there is a log­nor­mal dis­tri­bu­tion of ob­served pre­da­tion suc­cess among in­di­vi­d­ual cats, with a few in­di­vi­d­u­als act­ing as “su­per-preda­tors”, and a much larger num­ber cap­tur­ing very few or no prey. Lloyd et al. (2013b) re­port that of their group of 55 cats, one in­di­vi­d­ual cap­tured five prey, and 3 cap­tured 4, while the ma­jor­ity cap­tured one to no prey. Similarly, in a study in Switzer­land, five cats ac­counted for 75% of prey and 11 cats did not re­turn any prey items (Tschanz et al. 2011).

Loss et al. (2013) cite three refer­ences for their use of a uniform dis­tri­bu­tion be­tween 0.5 and 0.8 to de­scribe the per­centage of owned cats that hunt (Bar­ratt et al. 1998, Fiore un­pub, Crooks & Soule 1999), while Blancher (2013) as­sumes that all out­door cats hunt suc­cess­fully. This fac­tor may be a great im­por­tance when we con­sider that, by us­ing the mean pre­da­tion rate to sum­ma­rize data in the pub­lished stud­ies, we may have over­es­ti­mated the av­er­age pre­da­tion rate per cat (be­cause of the in­fluence of the long tail of the dis­tri­bu­tion on the mean). Bar­ratt et al. (1998) note that us­ing the mean in­stead of the me­dian will dou­ble the es­ti­mated per cat pre­da­tion rate. Fur­ther, when con­struct­ing simu­la­tions Loss et al. (2013) and Blancher (2013) use a uniform dis­tri­bu­tion to de­scribe pos­si­ble pre­da­tion rates, which as­sumes that both large and small rates are equally prob­a­ble: an un­likely sce­nario given what we know about pre­da­tion rates.

Prey re­turn rate

The prey re­turn rate is a cor­rec­tion fac­tor de­signed to ac­count for the fact that pre­da­tion rates es­ti­mated from the num­ber of prey re­turned to the own­ers only rep­re­sents a por­tion of the to­tal kills. Next to the num­ber of un­owned cats, this is the model pa­ram­e­ter for which we have the least in­for­ma­tion. More­over, a closer ex­am­i­na­tion of the data in­di­cates that the use of this pa­ram­e­ter will gen­er­ate max­i­mum owned cat pre­da­tion rates greater than the max­i­mum un­owned cat pre­da­tion rates in the origi­nal Loss et al. model (2013) based on EU and US data, and the same is true for the ru­ral cat pre­da­tion rate in the Blancher (2013) model, pos­si­bly be­cause in both cases the cor­rec­tion fac­tor is used across a range se­lected from data that in­cluded scats and stom­ach con­tents as well as prey re­turns.

Loss et al. (2013) use a uniform dis­tri­bu­tion be­tween 1.2 to 3.3 to es­ti­mate of this pa­ram­e­ter, based on three stud­ies. Wolf (2016) notes that the low­est es­ti­mate (from Ge­orge 1974) is ac­tu­ally based on a mis­read­ing. The au­thor of that pa­per is ac­count­ing for lack of con­tin­u­ous ob­ser­va­tion of cat re­turn rates by the hu­man owner rather than rep­re­sent­ing a differ­ence be­tween prey cap­tures and prey re­turns. The top of the range used by Loss et al. (2013) is based on a pre­da­tion rate calcu­lated from 4 suc­cess­ful mam­mal cap­tures noted in 181 to­tal hours of ob­ser­va­tions of 12 cats in New York (Kays & De­wan 2004), and com­par­ing the ex­trap­o­lated pre­da­tion rate to prey re­turns. Blancher (2013) also use a mis­read­ing for the Ge­orge (1974) study for the bot­tom of a uniform dis­tri­bu­tion rang­ing from 1.2 to 5.8 for this pa­ram­e­ter. They note the un­satis­fac­tory sam­ple size from Kays & De­wan (2004), and re­port on an un­pub­lished manuscript by Fiore and Sul­li­van, where the au­thors an­a­lyzed scat sam­ples to show that 21% of the time, cats in­gested birds with­out owner knowl­edge, data from Loyd et al. (2013b) from video cam­eras on cats (n = 39) and found that pet cats brought home 23% of prey. Krauze-Gryz et al. (2012) found that pre­da­tion rate for all prey types was 11.4 times higher when based on scats and stom­ach sam­ples than from prey re­turns (in­clud­ing in­sect con­sump­tion). While the pub­lished pa­per re­ports no sig­nifi­cant differ­ence be­tween prey re­turns and con­sump­tion for birds, Blancher (2013) states “their raw data for birds sug­gest a 5.8x ad­just­ment”, and use this value as the max­i­mum of a uniform dis­tri­bu­tion from 1.2 to 5.8 for both birds.


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This es­say is a pro­ject of Re­think Pri­ori­ties.

It was writ­ten by Kim Cud­ding­ton with con­tri­bu­tions from Derek Foster and David Moss. Jane Capozzelli, Mar­cus Davis, Michelle Gra­ham, Peter Hur­ford, Si­mon Eck­er­ström Lied­holm, Abra­ham Rowe, Ja­son Schukraft, Daniela R. Wald­horn, and Saulius Šimčikas pro­vided helpful com­ments on this es­say.

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