There is a great book out that I have just started titled " Living on the
Wind Across the Hemisphere with Migratory Birds" by Scott Weidensaul. I
am only 1/8th of the way through it, but I have learned that migration
existed before the ice age, and that there are many theories behind
migration. Primarily, birds migrate to seek a better food supply. While
global warming may expand suitable habitat for a given species, another
part of the equation has to deal with how birds know where to go, and by
what route.For example, rufous hummingbirds are now showing up more
routinely in the gulf states in the non-breeding season, presumably
because of a better food supply. If global warming was creating better
habitat, why then aren't Anna's hummingbirds showing up in the gulf
states in the same proportion as Rufous? Another example is the Northern
wheatear, which breeds in Alaska, the Yukon, and the eastern Canadian
Arctic south to Labrador. "In autumn, the Eastern population crosses
Baffin Bay to southern Greenland, then waits for northwest winds to make
a single-stage flight over Iceland and Britain, down the Iberian
Peninsula to Gibralter, Finally, they cross the Sahara to the tropical
savannas of West Africa's bulge, having flown nearly 5,000 miles" These
Alaskan birds, on the other hand, migrate west across the Bering sea,
where they join Siberian wheatears in trekking across Russia before
angling south through Turkey and Syria. They cross the Sinai and the Red
Sea into East Africa and fetch up for the winter in the grasslands of
Tanzania, a trip of nearly 7,000 miles". The author explains that this
migration started after the last ice age, and was a result of an
expanding range of the wheatear, though it still had ties to the
non-breeding grounds. He also points out the physical differences in the
populations- namely that the African birds have short, stubby wings,
European wheatears have longer wings, while the Canadian wheatears have
long, pointed wings.
Anyway, its a fascinating book.
-Jamie

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