Cat predation on wildlife
Cat predation on wildlife is the result of the natural instincts and behavior of both feral and domesticated cats to hunt small prey, including wildlife. This is sometimes seen as a desirable phenomenon, such as in the case of barn cats and other cats kept for the purposes of pest control. As an invasive species and superpredator, they do considerable ecological damage. In Australia, hunting by cats helped to drive at least 20 native mammals to extinction, and continues to threaten at least 124 more. Their introduction has caused the extinction of at least 33 endemic species on islands throughout the world. Feral and domestic cats kill billions of birds in the United States every year, where songbird populations continue to decline.

Birds
A 2013 study by Scott R. Loss and others of the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service found that free-ranging domestic cats (mostly unowned) are the top human-caused threat to wildlife in the United States, killing an estimated 1.3 to 3.7 billion birds and 6.3 to 22.3 billion mammals annually. These figures were much higher than previous estimates for the U.S. Unspecified species of birds native to the U.S. and mammals including mice, shrews, voles, squirrels and rabbits were considered most likely to be preyed upon by cats.

Perhaps the first U.S. study that pointed to predation by cats on wildlife as a concern was ornithologist Edward Howe Forbush's 1916 report for the Massachusetts State Board of Agriculture, The Domestic Cat: Bird Killer, Mouser and Destroyer of Wildlife: Means of Utilizing and Controlling It.

The Royal Society for the Protection of Birds states that there is no scientific evidence that cat predation "is having any impact on bird populations UK-wide." 

Island settings pose particular challenges for wildlife. A 2001 study identified cats alone as responsible for the plight of some island bird species, such as the Townsend's shearwater, socorro dove, and the Marquesan ground dove. The same study identified the greatest cause of endangerment of birds as habitat loss and degradation, with at least 52% of endangered birds affected, while introduced species on islands, such as domestic cats, rats and mustelids, affected only 6% of endangered birds. Other studies caution that removing domestic cats from islands can have unintended consequences, as increasing rat populations can put native bird and mammal species at risk.

Impact by location

Australia

Cats in Australia have been found to have European origins.  This is important to note because of their effect on native species. Feral cats in Australia have been linked to the decline and extinction of various native animals. They have been shown to cause a significant impact on ground nesting birds and small native mammals.

Feral cats have also hampered any attempts to re-introduce threatened species back into areas where they have become extinct as the cats have hunted and killed the newly released animals. Numerous Australian environmentalists claim the feral cat has been an ecological disaster in Australia, inhabiting most ecosystems except dense rainforest, and being implicated in the extinction of several marsupial and placental mammal species. Some inhabitants have begun eating cat meat to mitigate the harm that wild cats do to the local wildlife.

New Zealand

The fauna of New Zealand has evolved in isolation for millions of years without the presence of mammals (apart from a few bat species). Consequently, birds dominated the niches occupied by mammals and many became flightless. The introduction of mammals after settlement by Māori from about the 12th century had a huge effect on indigenous biodiversity. European explorers and settlers brought cats on their ships and the presence of feral cats was recorded from the latter decades of the 19th century.
It is estimated that feral cats have been responsible for the extinction of six endemic bird species and over 70 localised subspecies as well as depleting bird and lizard species.

United Kingdom
In the U.K., The Royal Society for the Protection of Birds states that there is no scientific evidence that cat predation "is having any impact on bird populations UK-wide. The article goes on to say, "This may be surprising, but many millions of birds die naturally every year, mainly through starvation, disease, or other forms of predation. There is evidence that cats tend to take weak or sickly birds".

"Those bird species that have undergone the most serious population declines in the UK (such as skylarks, tree sparrows and corn buntings) rarely encounter cats, so cats cannot be causing their declines. Research shows that these declines are usually caused by habitat change or loss, particularly on farmland." This evidence is despite the common practice in the U.K. of allowing owned cats access to the outdoors, which is recommended to prevent feline obesity (p. 138) and behavior problems and other health problems arising from confinement stress (p. 121).

SongBird Survival considers that "The prevailing line that "there is no scientific evidence that predation by cats is having any impact on bird populations in UK" is simply no longer tenable", noting "No study has ever examined the impact of cats on songbirds at the population level; Evidence shows that the recovering sparrowhawk population in the 1970-80s resulted in the decline of some songbird populations; Cats kill around 3 times as many songbirds as sparrowhawks; The mere presence of cats near birds' nests was found to decrease provision of food by a third while the resultant mobbing clamour from parent birds led in turn to increased nest predation by crows and magpies; [and that] It is therefore far more likely that cats have an even greater impact on songbird populations than sparrowhawks."

Sir David Attenborough in his Christmas Day, 2013, edition of BBC Radio 4 programme Tweet Of The Day said ""Cats kill an extraordinarily high number of birds in British gardens". Asked whether cat owners should buy bell collars for their pets at Christmas, he replied: "That would be good for the robins, yes."

Islands

Consequences of introduction

Many islands host ecologically naive animal species. That is, animals that do not have predator responses for dealing with predators such as cats. Feral cats introduced to such islands have had a devastating impact on these islands' biodiversity.

They have been implicated in the extinction of several species and local extinctions, such as the hutias from the Caribbean, the Guadalupe storm petrel from the Pacific coast of Mexico, and Lyall's wren. In a statistical study, they were a significant cause for the extinction of 40% of the species studied. Moors and Atkinson wrote, in 1984, "No other alien predator has had such a universally damaging effect."

Feral cats, along with rabbits, some sea birds, and sheep, form the entire large animal population of the remote Kerguelen Islands in the southern Indian Ocean. Although exotic mammals form the bulk of their diet, cats' impact on seabirds is very important.

Restoration
Because of the damage cats cause in islands and some ecosystems, many conservationists working in the field of island restoration have worked to remove feral cats. (Island restoration involves the removal of introduced species and reintroducing native species.) , 48 islands have had their feral cat populations eradicated, including New Zealand's network of offshore island bird reserves and Australia's Macquarie Island.

Larger projects have also been undertaken, including their complete removal from Ascension Island. The cats, introduced in the 19th century, caused a collapse in populations of nesting seabirds. The project to remove them from the island began in 2002, and the island was cleared of cats by 2004. Since then, seven species of seabird that had not nested on the island for 100 years have returned.

In some cases, the removal of cats had unintended consequences. An example is Macquarie Island, where the removal of cats caused an explosion in the number of rabbits, rats, and mice that harm native seabirds even if the eradication was positioned within an integrated pest management framework. The removal of the rats and rabbits was scheduled for 2007 and it could take up to seven years and cost $24 million.

Mice and rats

Cats are sometimes intentionally released into urban environments on the popular assumption that they will control the rat population; but there is little scientific basis for this. The reality is that cats find rats to be large and formidable prey, and so they preferentially hunt defenseless wildlife such as lizards and songbirds instead. Scientists and conservationists oppose the use of cats as a form of rodent control because they are so inefficient at destroying pest species that the harm they do to native species in the process outweighs any benefit.    

In 2002, feral cats introduced to a flower market in Los Angeles, California, were noted to have helped lower rat populations. In Chicago's 47th Ward, feral cats were introduced in 2012 in the hope that it would help a rat problem there.