> From its foggy depths my memory insists this is probably NOT the case -
> rather for several N temperate migrants the more common pattern is that
> birds from the more Nly part of the wintering range migrate to the
> more Sly part of the breeding range. And the N part of the breeding
> range is later filled by birds that have come farther. Can not recall
> any citations to back this up (or even where or when I acquired the idea).

One place you can find discussion of this is in the Handbook of American
Birds by (I'm not home...) Palmer??? Anyway, he discusses this in his
chapter on red-tailed hawks, which follow that pattern in at least
part of their range.

-Don Baccus-