How do you actually test that as a working ecologist, though? Given two geographically and somewhat morphologically district warblers (for example) you would ne

How do you actually test that as a working ecologist, though? Given two geographically and somewhat morphologically district warblers (for example) you would need to convince two of them to breed (after capturing at least one) and then check viability of an offspring. It's not terribly practical... Or, in this case with ancient human remains, there's no way to check viability when all subjects have been dead for a hundred thousand years.