A study by British scientists has documented significant new impacts to birds from outdoor cats. According to the report, even brief cat appearances near bird-nest sites at least doubles the number of kills to eggs and young birds and induces parent-bird behavioral changes that lead to an approximately 33-percent reduction in the amount of food brought to nestlings following a predation threat.
Scientists observed 47 blackbird nests in 2010 and 49 nests in 2011 in Sheffield, England, during the March to August breeding season and compared nest dynamics following presentation of a taxidermist-prepared cat, predatory grey squirrel and rabbit. The crucial finding is that the natural response of parenting birds to the appearance of predators—alarm calling and nest defense—alerts third-party predators to the nests, dramatically affecting predation rates. In addition, the threat of cat predation results in lower feeding rates of young birds for prolonged periods.
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 says. Predation by third-party animals during chick incubation was highest following presentation of the cat (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 and 0 percent for the other two animals. At the old-chick stage, there was no predation, as young birds could escape on their own.
Even more concerning, the study found no evidence that parental feeding rates returned to normal even after the cat had been removed for lengthy periods of time, up to 90 minutes later. Furthermore, no evidence suggested that the parent birds compensated for the reduced feed 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 says.
Behavioral changes in birds caused by a perceived cat threat "may have considerable implications for (bird) population and community dynamics,” the study notes, and it 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 permanently housing cats indoors is the most effective option for bird management.
"Here we have yet another peer-reviewed study that documents additional, serious impacts to bird populations that previously have not been fully appreciated.” says Clare Nielsen, communications director for the American Bird Conservancy, a U.S. bird-conservation organization. "Feral and outdoor cats are simply devastating populations of birds and other wildlife.”
The study, which was peer-reviewed and published in the Journal of Applied Ecology, follows the release ofa widely-reported study from the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service that found 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 1.4 to 3.7 billion and mammal mortality likely 6.9 to 20.7 billion.