An ornithologist has observed two species of nuthatch that appear to live in similar environments: both species forage for insects on the same trees, but one species forages in the upper canopy, and the other species forages in the lower canopy. This is probably an example of:
A) intraspecific competition that leads to habitat separation. B) territorial competition leading to niche overlap. C) territorial competition leading to competitive exclusion. D) coexistence through resource partitioning.