On the hunt, cat and great tits in winter landscape (1881), by Bruno Liljefors (Wikimedia Commons)

While there are rare (and very cute) exceptions, cats and birds do not get along. Cats are predatory by nature; their hunting instinct never goes away. Birds are one of their primary targets—and the fatality statistics are staggering. “There are now over 100 million free-roaming cats in the United States,” according to NYC Bird Alliance (formerly known as NYC Audubon), a nonprofit bird advocacy group. “[T]hey kill approximately 2.4 billion birds every year in the U.S. alone, making them the single greatest source of human-caused mortality for birds.” (The other leading killer of birds is also human-caused: window strikes kill as many as one billion birds in the U.S. every year, according to the American Bird Conservancy. Ornithologist Daniel Klem Jr. of Muhlenberg College puts the figure somewhere between 1.28 and 5.19 billion.)

Feral and free-roaming pet cats pose a grave threat to wild bird populations around the globe, with significant ecological consequences. The toll cats take on birds—through direct predation, stress induction, and disruption of nesting behavior—is increasingly well-documented by scientists and conservationists.

“When outside, cats are [an] invasive species that kill birds, reptiles, and other wildlife,” NYC Bird Alliance points out. “But despite being fed, they kill wild birds and other animals by instinct.” Moreover, the domestication of cats has allowed the species to spread and thrive in many regions it might not have otherwise been able to inhabit. However, while the scope of the issue is vast and the ecological consequences are grave, solutions exist to mitigate this ongoing and expanding environmental crisis.

A Global Eco-Crisis

Estimates suggest that domestic cats (Felis catus) might be killing billions of birds each year. A major 2013 study by the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service concluded that free-ranging domestic cats kill between 1.3 and 4 billion birds annually in the United States alone. (This figure accounts for both feral cats and free-roaming pet cats, with the majority of the bird deaths attributed to cats without human guardians, which includes those cats in feral colonies, also known as community cats.)

The global picture is similarly grim. In a 2017 paper published in Frontiers in Ecology and the Environment, ecologist Scott Loss of the Oklahoma State University, and Peter Marra, the former director of the Smithsonian Migratory Bird Center (who co-authored the abovementioned 2013 study with Tom Will of the U.S. Fish and Wildlife Service), assert that domestic cats have “contributed to at least 63 vertebrate extinctions, pose a major hazard to threatened vertebrates worldwide, and transmit multiple zoonotic diseases.” They point out that “[m]ore than a dozen observational studies, as well as experimental research, provide unequivocal evidence that cats are capable of affecting multiple population-level processes among mainland vertebrates.”

In countries like Australia and New Zealand, where many native species evolved without mammalian predators, the impact of introduced cats has been particularly catastrophic. Numerous species of birds, like the piping plover, as well as small mammals and reptiles, have been driven to extinction or near extinction due to cat predation. Islands are especially vulnerable, as their ecosystems tend to be isolated and finely balanced.

How Cats Affect Bird Populations

The primary threat cats pose to birds is direct predation. Birds are particularly vulnerable during breeding season, when they are tied to specific territories and may have limited mobility while incubating eggs or feeding young. Ground-nesting birds are at incredibly high risk, as they often rely on camouflage and stillness rather than flight to evade threats—tactics that are ineffective against cats’ stealthy and persistent hunting methods.

Birds fortunate enough to evade capture still suffer being in the proximity of outdoor cats. Research indicates that the mere presence of cats can cause stress to birds, impacting their reproductive success and leading to adverse behavioral changes such as increased vigilance, which results in reduced feeding rates and less effective parenting. A 2013 British study published in the Journal of Applied Ecology found that birds may avoid returning to their nests or dens for extended periods to prevent leading predators, such as cats, from getting to their young. This avoidance behavior, driven by the stress of a nearby predator, can reduce the growth rate of young birds by approximately 40 percent. In some cases, birds may abandon nests altogether if they sense persistent danger, especially from cats that return to the same area regularly.

By dramatically reducing bird populations, cat predation can also negatively impact plant pollination, forest regeneration, and human health—all of which have detrimental economic consequences. Trophic cascades may even be triggered, causing adverse effects up and down the food chain. In a study published in 2024 in Nature Climate Change, researchers led by ecologist Daisy Dent at the Crowther Lab at ETH Zurich, a Swiss university, showed that when wild birds move freely........

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