The species are birds, some have larger wings or smaller wings, and their feathers are either black or grey. the Birds with smaller wings tend to have grey feathers, and the birds with black feathers more often have larger wings
The Birds with larger wings cannot live in the trees because it's too crowded and they can't fly because of all the branches. The ones with smaller wings however, can live in the trees because their wings don't take up too much space.
The trees grow berries that the smaller winged birds can eat, but the larger winged birds can't reach them, so the larger winged birds have to find something else to eat.