Sex bias on cat predation on Eastern bluebirds

Pitts, T. D., & Snow, R. W. (1996). Mortality of Banded Adult Eastern Bluebirds. Audubon Society of Omaha.

Domestic cats are, narrowly, the second most important cause of non-natural mortality, just after shooting. 

Of 51 Eastern Bluebirds killed by domestic cats, 19 were females and 5 were males; the sex of the other birds was not indicated. While this sample size is not much larger than that of bluebirds found dead in the nest (24 versus 16), the trend seen here shows a significant difference in the number of males and females killed (X² = 8.16, df = 1, P <0.005). Figure 5 shows that mortality due to domestic cats is highest during the nesting season. Considering that the female bluebirds, who construct each nest, lay eggs, and incubate the eggs, receive significant amounts of help from the male only when caring for nestlings, we are not surprised that more females are captured by domestic cats. 

(Domestic cats kill an unknown, but probably large, number of nestling bluebirds. However, since this paper deals only with adult mortality we have not explored the effects of cat predation on nestlings.)

Predation by cats, the largest human related source of bird mortality in Canada

Blancher, P. 2013. Estimated number of birds killed by house cats (Felis catus) in Canada. Avian Conservation and Ecology 8(2): 3.

Predation by house cats (Felis catus) is one of the largest human-related sources of mortality for wild birds in the United States and elsewhere, and has been implicated in extinctions and population declines of several species. However, relatively little is known about this topic in Canada. The objectives of this study were to provide plausible estimates for the number of birds killed by house cats in Canada, identify information that would help improve those estimates, and identify species potentially vulnerable to population impacts. In total, cats are estimated to kill between 100 and 350 million birds per year in Canada (> 95% of estimates were in this range), with the majority likely to be killed by feral cats. This range of estimates is based on surveys indicating that Canadians own about 8.5 million pet cats, a rough approximation of 1.4 to 4.2 million feral cats, and literature values of predation rates from studies conducted elsewhere. Reliability of the total kill estimate would be improved most by better knowledge of feral cat numbers and diet in Canada, though any data on birds killed by cats in Canada would be helpful. These estimates suggest that 2-7% of birds in southern Canada are killed by cats per year. Even at the low end, predation by house cats is probably the largest human-related source of bird mortality in Canada. Many species of birds are potentially vulnerable to at least local population impacts in southern Canada, by virtue of nesting or feeding on or near ground level, and habitat choices that bring them into contact with human-dominated landscapes where cats are abundant. Because cat predation is likely to remain a primary source of bird mortality in Canada for some time, this issue needs more scientific attention in Canada.

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La prédation par les chats domestiques (Felis catus) est une des causes de mortalité liées aux humains les plus importantes chez les oiseaux sauvages aux États-Unis et ailleurs; elle a également été mise en cause dans l’extinction et le déclin de populations de plusieurs espèces. Toutefois, la situation au Canada est plutôt méconnue. Cette étude visait à obtenir des estimations plausibles du nombre d’oiseaux tués par les chats domestiques au Canada, à déterminer les informations additionnelles pouvant contribuer à améliorer ces estimations et enfin, à identifier les espèces dont les populations pourraient être touchées. Dans l’ensemble, nous avons estimé que les chats tuaient entre 100 et 350 millions d’oiseaux par année au Canada (> 95 % des estimations se trouvaient dans cette plage), les chats sauvages tuant vraisemblablement la majorité de ces oiseaux. Cette plage d’estimations est fondée sur des relevés indiquant que les Canadiens possèdent 8,5 millions de chats domestiques environ, une approximation grossière du nombre de chats sauvages se situant entre 1,4 et 4,2 millions et des taux de prédation tirés d’autres recherches menées ailleurs qu’au Canada. Une meilleure connaissance du nombre de chats sauvages et de leur régime alimentaire au Canada permettrait d’améliorer grandement la fiabilité de l’estimation du nombre d’oiseaux tués, bien que n’importe quelle donnée sur les oiseaux tués par les chats au Canada y contribuerait également. Nos estimations indiquent que de 2 à 7 % des oiseaux dans le sud du Canada sont tués par les chats chaque année. Même en considérant l’estimation la plus prudente, la prédation par les chats domestiques est probablement la cause de mortalité d’oiseaux liée aux humains la plus importante au Canada. De nombreuses espèces d’oiseaux sont potentiellement vulnérables à des effets sur leurs populations au moins localement dans le sud du Canada, en raison de leur nidification ou de leur alimentation au sol ou près du niveau du sol, et des choix d’habitats qui les amènent dans des paysages dominés par les humains où les chats sont abondants. Étant donné que la prédation par les chats demeurera vraisemblablement une source importante de mortalité d’oiseaux au Canada dans un avenir rapproché, cet enjeu mérite davantage d’attention scientifique au Canada.

Cats as first cause of death of British ringed birds

The BTO (British Trust for Ornithology) started bird ringing 100 years ago, in 1909. To date some 36 million birds have been ringed, providing a unique insight into the birds of Britain and Ireland...

...The BTO receives reports of around 14,000 birds with rings every year, providing an invaluable source of information on the latest trends in bird conservation. I think that one of the telling statistics is the causes of death. For all those people who get very upset when birds are caught in oil spills, (5,600 records), collisions with windows (13,800) or collisions with overhead wires (9,400), all these numbers are dwarfed by the 41,900 birds killed by domestic cats.

New evaluation of cat's kill in US: probably tens of billions kills per year

Loss, S.R., T. Will & P.P. Marra. 2013. The impact of free-ranging domestic cats on wildlife of the United States. Nature Communications 4, 1396 doi:10.1038/ncomms2380

Anthropogenic threats, such as collisions with man-made structures, vehicles, poisoning and predation by domestic pets, combine to kill billions of wildlife annually. Free-ranging domestic cats have been introduced globally and have contributed to multiple wildlife extinctions on islands. The magnitude of mortality they cause in mainland areas remains speculative, with large-scale estimates based on non-systematic analyses and little consideration of scientific data. Here we conduct a systematic review and quantitatively estimate mortality caused by cats in the United States. We estimate that free-ranging domestic cats kill 1.4–3.7 billion birds and 6.9–20.7 billion mammals annually. Un-owned cats, as opposed to owned pets, cause the majority of this mortality. Our findings suggest that free-ranging cats cause substantially greater wildlife mortality than previously thought and are likely the single greatest source of anthropogenic mortality for US birds and mammals. Scientifically sound conservation and policy intervention is needed to reduce this impact.

See also this entry.


Read more national surveys results on cat predation