>A couple of questions occur, namely
>1. It would seem that birds in the northern part of the breeding range
> winter in the northern part of the wintering range, and a similar
> case for southern birds.
> Is this documented ?
>
>2. Do northern and southern birds both begin migrating at the same time ?
>
>Neil J. Fergusson

1) Actually, it's more common for birds in the northern part of the
breeding range to winter in the *southern* part of the winter range. This
phenomenon, called "leapfrog migration," has been documented for many
species, commonly by analysis of the wintering grounds of various
subspecies. The northernmost populations may be highly migratory, the
middle ones less highly migratory, the southernmost ones resident. Their
distribution with respect to each other is exactly reversed in winter.

It has been said to be a mechanism for avoidance of competition between
different populations of a species, but there isn't anything in the way of
experimental evidence for this.

2) Basically birds that breed farther north migrate later than those that
breed farther south, all other things being equal (which, of course,
they're not always). An example: Common Loons, which breed in the boreal
forest, acquire breeding plumage in March, and most leave in April.
Pacific and Red-throated loons, which breed in the Arctic, acquire breeding
plumage in April and leave in May.

Dennis Paulson, Director phone: (206) 756-3798
Slater Museum of Natural History fax: (206) 756-3352
University of Puget Sound e-mail: dpaulson at ups.edu
Tacoma, WA 98416