Why is territoriality an adaptive behavior for songbirds maintaining populations at or near their carrying capacity?

A) Songbirds expend a tremendous amount of energy defending territories so that they spend less time feeding their young and fledgling mortality increases.
B) Only the fittest males defend territories and they attract the fittest females so the best genes are conveyed to the next generation.
C) Songbird males defend territories commensurate with the size from which they can derive adequate resources for themselves, their mate, and their chicks.
D) Many individuals are killed in the agonistic behaviors that go along with territorial defense.
E) Adult songbirds make improvements to the territories they inhabit so that they can produce successfully fledged chicks.