Answer:
The correct answer is testing of their DNAs.
Step-by-step 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.