After wild animals die....

After wild animals die, where are those dead bodies?

They're hundreds of thousand birds, squirrels near the place I live.
But I never saw any one of the corpse of them except for the ones
killed by car accidents.

Do those wild animals somehow know they're going to die when they're
really old/sick? And then they go to a special place waitting for the
call of death? Or do they bury their fellows? At lease flying birds
never die when they're flying, do they?
 
This problem might be a trivial one, but it has been haunting in my
mind for quite a long time. :-)