What piece of evidence would go against the argument and suggest that birds are actually different species

The birds have different mating seasons would answer the question whether the two bird species are actually the same species or just two different ones. The reason for this is that when two bird species have different mating seasons this actually tells us that they most likely don't mate with each other and for that reason it wouldn't be really possible for them to be one species. 

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it should be briana that has it to me