What do wild birds eat?

If it is edible, there will be a bird somewhere that eats it. The only exception is general plant matter, like grass and such. Mature plant matter consists mostly of cellulose and to be able to eat mainly cellulose, an animal needs to ferment it to make it digestible. Sufficient body-size to allow fermentation is incompatible with flight, so only flightless birds use fermentation, with the exception of the Hoatzin, which is a very poor flyer. Even so, there are no general herbivorous birds equivalent to mammals such as deer, sheep and cattle.

Many birds eat flowers and buds, but these contain quite a bit of nutrient apart from cellulose.