I agree with most of that. The term evolutionarily significant unit admits a whole lot of cases to consideration without the vagueness that attends the word species. Whether redpoll types are species, subspecies or balanced polymorphisms, they are ESUs.
The quibble over whether redpolls are 1, 2 or 3 species is not about whether they are ESUs. It is about how to apply a vaguely defined set of terms to the units. It certainly appears to be an argument about language more than biology.
Honestly it concerns me how many people are “against” subspecies, and rather just lump everything into single species. Subspecies are inherently designed to classify “near species”, and recognize diversity that is more variable and minimal. There is never pressure to identify to that level, anyway. But their existence shouldn’t always be dismissed.
I do really hope that the redpolls become subspecies, unless there is truly evidence suggesting otherwise (like breeding subspecies A in the conditions that subspecies B lives in, and having the “wrong” subspecies appear in the offspring).
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