Some birds eat worms. Some birds eat breadcrumbs. Some birds eat berries. And some birds eat cat food. The blackbird who lives in our garden, for instance.
He's quite clever, too. He only goes to eat from the cats' food bowl when all three of our cats are inside the house. (And watching him from the glass door, and loudly begging to be let outside so they can go kill the insolent bird - which of course we won't let them do.) It's great.
(I wonder if he'll ever get caught and eaten by a cat? And if yes, will the cat wonder why the bird tastes just like cat food?)