northerly migrants reach the common breeding ground earlier and mate with one another before southerly migrants arrive. This difference in timing may one day drive the two populations to become two species.
Well then, that settles it! Speciation occurs, so stuff it ID'ers!
On a related note, Japanese people and myself are different species 'cause I can't afford to go to Japan and mate with them and that exchange student spurned my drunken advances.
Owl_Eagle(If what I just wrote makes you sad or angry,
LOL ;-)