Domestic cats hunt and kill birds all the time, as do feral housecats living in urban and rural environments, and while domestic cats do not always eat their kills, feral housecats nearly always do. Cats are considered extremely dangerous to local ecosystems and can have a significant impact on wildlife populations because they hunt for sport, not just for sustenance.
Domestic housecats will hunt for sport, not just to acquire nutrients, and thus can kill for fun when not hungry. This drives up the number of kills a cat makes in a given period and can lead to significant changes in local ecology.
Cats kill nearly 4 billion birds and over 20 billion small mammals like squirrels and mice a year, the Huffington Post estimates. Wild housecats, bred in the wild or escaped from domestic life, kill significantly more often than their tame counterparts because they must kill in order to feed themselves and their kittens.
There are over eighty million feral cats living in the Continental United States. An average cat can spend up to a third of its day pursuing and killing prey to feed or entertain itself, and their prodigious numbers and constant hunting combine to make them deadly and efficient killers of birds, mice and other small animals.