Depends on the bird species. Most song birds eat seeds and some bugs. Some birds like kingfishers eat small fish from ponds and lakes. Other birds like hawks and owls will eat squirrels, mice, rats and rabbits.
Seeds , bugs
Some wild birds eat worms.
If you want to feed birds in your yard I'd use some wild bird seed or old bread,popcorn,suet,leftovers even. I've seen them eat a variety of foods.