There is strong evidence that two species of cliff-dwelling birds have nested in the same environment and fed in the same water for thousands of years, with each having stable population sizes. Based on this information, which of the following statements is the best conclusion?
(A) One species is a better diver than the other species is.
(B) The two birds compete for food
(C) One species will displace the other
(D) The two birds have different ecological niches that allow coexistence
(E) One species eats a more
Answer: D