On Mt. Desert Island, hay-breasted warblers are only common where there are dense growths of lichen-covered lower branches of spruce in the shade of the forest crown. It is in this zone that a large part of their activity takes place and here
their sluggish, radially moving, feeding behavior is well suited. This habitat appears when the forest becomes dense and has large trees. Consequently, the bay-breasted warblers only remained permanently in plots 0 and E. (Two set up territories in plot C but had apparently left by June 8.) Again it appears that this habitat is occupied
by the bay-breasted partly because it is not occupied by the black-throated green. In the bud ,worm infested spruce and balsam stands near Ft. Kent, Maine, the bay-breasted warbler was the dominant bird; here they occupied the dense young
stands of conifers and black-throated greens were forced to occupy the ridges covered with mixed growths of hemlock and hardwoods. The type of competition in heterogeneous regions mentioned in the first section provides an appropriate explanation for the change in dominance; the forest composition of the whole region is more important
than the very local conditions.