While in South America, You Come Across What You Think

While in South America, you come across what you think are two groups of birds in the same location. They are nearly identical aside from their color. After years of observation, you conclude that the birds eat similar diets and share similar behaviors but do not reproduce with each other. These groups of birds appear to be an example of:

A) a single biological species.
B) a single ecological species.
C) ring species.
D) two different species on the basis of the ecological niche occupied.
E) two different species on the basis of reproductive behavior.