Your cat does not realize that it is a cat. After years and years of domestication your cat has forgotten that it is a predator. A cat no longer has to rely on its hunting and stealth abilities to acquire food, now they just wait for the sound of a can opener or the click of an automatic feeder. When a cat comes across a cute little bird it sees a new friend and not a yummy snack.
Disclaimer: We do not approve of cat on bird violence, so we suggest keeping your animals separated just in case your cat is lower on the evolutionary ladder.