[u2-l2] two birds that look very similar to each other have always been considered two distinct species based on differences in their body size and in where they live. a group of ornithologists studying the two species begins to think they might not be separate species based on observed behavior. what would be the best next step regarding this new hypothesis?

The correct answer is testing of their DNAs.

Explanation:

In my opinion, the next step considering the mentioned hypothesis in the question is to perform DNA examinations on the two species in order to observe how genetically identical they are.  

Prior to making any kind of conclusions, like publishing the ideas in a scientific journal, there is a need to ensure that the hypothesis is correct. Thus, comparing the DNAs of the two birds in order to access their similarities and differences would be an ideal way to test the hypothesis.