A stand of Douglas fir trees is home to 3 similar species of small insect-eating birds. They eat a variety of insect species: one insect burrows into the tree bark, one lives in the tree needles (leaves), and one flies between trees. All three bird species are able to eat any of the insect species.
If one bird species eats only insects in the tree bark, another forages only in the needles, and the third only catches insects as they fly between trees, then the birds are experiencing:

A. resource partitioning.
B. character displacement.
C. mimicry.
D. predation.
E. intraspecific competition.