Fair enough. Who determines how much difference is needed, though? I know the definition of species is up to interpretation, but who is the arbiter?
There is no arbiter, nor any universally agreed on definition. That's why different works will claim different species.
It certainly is. And Aiki says that the two populations can breed, but often don't because of the timing of their arrivals. Would a speciation event require them to be unable to interbreed, or is it enough to simply show that they tend not to?
Regardless of what people may claim their standard of speciation is, I think you'd be very hard pressed to find an actual list of species written by anyone which truy obeyed the rule that two species should be biologically incapable of breeding. For most species, I doubt anyone really knows, since I don't think all closely related species are routinely interbred to see if it produces fertile offspring. In cases where experiments have been done, intentionall or accidentally, some very distantly related species have been shown to be able to hybridise, and produce fertile offspring.
All sorts of fowl are capable of hybridising together, in many cases producing fertile offspring. There'd be far fewer species of grouse and duck if people were taking this concept seriously.
Using two different definitions of species,
a 1999 study counted either 101 or 249 endemic bird species in Mexico.
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