Two species of warbler are very similar. However one forages in the top of the trees while the other forages near the bottom. Which explains this difference?

I would say this is niche partitioning.

Explanation:

Every organism has a certain range of requirements called a niche. The competitive exclusion principle states that two organisms cannot occupy the exact same niche without one out-competing the other. Niche partitioning is basically changing some aspects of a niche so that there is no need for competition.