Outdoor cats kill birds. Lots of them. That’s why people often put bells on collars around a cat’s neck, but studies show that cats quickly learn how to hunt with the bell on anyways.
Now the latest study indicates that cats may hunt wildlife because they lack proper nutrition. Although commercial cat food may be formulated to give the cat everything it needs, such food may still lack crucial nutrients found only in wild animals.
Of course your cat may just like killing things and leaving the dead carcasses on the floor for you to step on.