Study Documents Dramatic New Impacts to Birds from Outdoor Cats

Cat with dead bird, iStockPhoto
Cat with dead bird by iStockPhoto

(Washington, D.C., April 18, 2013) A new study from British scientists has documented for the first time, significant new impacts to birds from outdoor cats, reporting that even brief appearances of cats near avian nest sites leads to at least a doubling in lethal nest predation of eggs and young birds by third-party animals, as well as behavioral changes in parent birds that lead to an approximately 33 percent reduction in the amount of food brought to nestlings following a predation threat.

The study was peer-reviewed and published in the Journal of Applied Ecology (January 30, 2013). The study was led by Karl Evans of the Department of Animal and Plant Sciences, University of Sheffield in collaboration with his PhD student Colin Bonnington and Kevin Gaston of the Environment and Sustainability Institute, University of Exeter.

The study was carried out by observing 47 blackbird nests in 2010 and 49 nests in 2011 in Sheffield, England, during the breeding season from March to August and compared nest dynamics following presentation of a taxidermist-prepared cat, a predatory grey squirrel, and a rabbit. The crucial finding is that the natural response of parenting birds to the appearance of predators – alarm calling and nest defense – dramatically affects rates of bird nest predation by third-party animals thusly alerted to the nest, as well as much lower feeding rates of young birds for prolonged periods following the threat of predation by cats.

The domestic cat model consistently prompted significantly higher alarm calling rates than either the rabbit or the squirrel. “Logistical models of nest fate demonstrated that the probability of nest predation within 24 hours of model exposure increased with the amount of parental nest defense,” the study said. Predation by third-party animals during chick incubation was highest following presentation of the cat model (23 percent of nests) followed by the grey squirrel (5 percent) and the rabbit (0 percent). At the young chick stage, predation was 13 percent for the cat model and zero for the other two models.  At the old chick stage, there was no predation owing to the ability of the young birds to escape on their own.

Even more concerning is the fact that the study found no evidence that parental feeding rates returned to normal even after the cat model had been removed for lengthy periods of time such as even up to 90 minutes later. Further, there was no evidence that the parents at any time compensated for the reduced feeding rate, by bringing more food at a later time.

“Reduced food delivery, even over short time periods, can adversely influence chick condition and reproductive success and over longer time periods can promote smaller clutches,” the study said.

The study said that the behavioral changes in birds caused by the appearance of cats “….may have considerable implications for (bird) population and community dynamics” and suggests that “…the impacts of sub lethal effects on avian prey populations are frequently greater than those arising from lethal effects…..”

The study concludes that whilst cats housed indoors require more care and attention from their owners the most effective management option is thus to house cats permanently indoors. About half of cat owners in North America do this to prevent cats having road traffic accidents or being injured in fights with other cats.

“Here we have yet another peer-reviewed study that documents additional, serious impacts to bird populations that previously have not been fully appreciated. Feral and outdoor cats are simply devastating populations of birds and other wildlife,” said Clare Nielsen, Director of Communications for American Bird Conservancy, one of the leading bird conservation organizations in the U.S.

The new study follows the release in January, 2013, of a new, widely-reported, peer-reviewed study by scientists from two of the world's leading science and wildlife organizations – the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service (FWS) – which found that bird and mammal mortality caused by outdoor cats in the United States is much higher than has been widely reported, with annual bird mortality now estimated to be 1.3 to 4.0 billion and mammal mortality likely 6.3 to 22.3 billion individuals.

That study's estimate of bird mortality far exceeds any previously estimated U.S. figure for cats. In fact, this magnitude of mortality may exceed all other direct sources of anthropogenic bird and mammal mortality combined. Other bird mortality sources include collisions with windows, buildings, communication towers, and vehicles, as well as pesticide poisoning.

The study estimated that the median number of birds killed by cats annually is 2.4 billion and the median number of mammals killed is 12.3 billion. About 69 percent of the bird mortality from cat predation and 89 percent of the mammal mortality was from un-owned, or feral, cats.

Free-ranging cats on islands have caused or contributed to 33 (14 percent) of the modern bird, mammal, and reptile extinctions recorded by the International Union for Conservation of Nature Red List of Threatened Species.


*This release was updated to reflect a revised estimate of bird and mammal mortality. Bird mortality changed from 1.4-3.7 billion to 1.3-4.0 billion, and mammal mortality changed from 6.9-20.7 billion to 6.3-22.3 billion.