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.
sometimes they will
Birds such as robins, cedar waxwings, and various species of thrushes are known to eat cranberries from the wild. These birds are attracted to the bright red color and tart taste of the berries.
Yes they sometimes eat.
poo
Yes many birds in the wild life eat each other
Nearly all domesticated birds do, but some wild birds eat berries and other things.
yes,many songbirds eat them.
Yes, wild birds can eat cranberries. Just last week, I put out some on my feeder and on the floor, and some birds eat it, and the squirrels and chipmunks all gobble up the rest on the floor.
yes
Yes
some do
Birds of prey, Kestrals for example, wild cats, fox.