Cats will hunt small wild animals, despite how well fed they are. Almost all young birds leave the nest before they can fly well and spend a day or two on the ground. As these fledglings are learning to fly, they are frequently caught by cats. Most of the birds caught by cats, but not killed outright, die of their injuries or infection. Cats that kill small rodents can eliminate a critical food source for owls and hawks.
Cars kill millions of cats each year.
Outdoor cats are exposed to serious and often fatal infectious diseases such as feline leukemia and rabies. Parasites such as fleas, ticks and intestinal worms pose a health threat to your cat. Some of these can be transmitted to humans. Cats outdoors are often chased by dogs or other cats, and killed, injured or hopelessly lost. Cats are often shot at, poisoned, trapped or tortured by neighbors annoyed by cats using their gardens as a litterbox or hunting birds and other small animals.