I and the Bird

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Tuesday, August 15, 2006

Loop Migrations

Are the same places I went to for the spring migration the same places I should go for the fall migration?

This question intrigues me more and more as I think about it. From a bird's perspective, a lot of these areas change a lot from spring to fall. Relying, as I have before, on the excellent migration of birds site from the Northern Prairie Wildlife Research Center, I've dug up some information on birds that do not follow the same migration route in the fall as they do in the spring.

It's called a "loop migration." Check out the NPWRC page here. Loop migrations have developed in a large number of unrelated species, from American Golden-Plovers to Connecticut Warblers, but scientists have not nailed down an exact cause or reason why one species would develop a loop and another would not.

There are a couple likely causes. Firstly, loop migrations could develop because, simply, the same route in the fall did not produce as much food for the birds as it did in the spring. Although reasons aren't given in the NPWRC article, I assume that birds that rely on food sources that change with the seasons (such as ripening berries or the horseshoe crab spawn) are more likely to develop loop migrations. What develops, then, is right from the pages of The Origin of Species: birds who follow the spring path back down in the fall will not find enough food and either die, fail to reproduce or just not follow that same route the next year. Once all those birds are weeded out a loop migration has developed.

A version of this food hypothesis is used to explain the loop migration of the American Golden-Plover, which migrates up through the middle of the U.S. in the spring and down the east coast in the fall. Scientists think that the risk of still-frozen soil and foggy conditions in the spring force Plovers to take a safer route through the middle of the country. In the fall, though, late-ripening berries in Nova Scotia and the Maritimes provide the birds with the energy needed for the long migration.

American Golden-Plover migration.  From the NPWRC.

Scientists provide another explanation for the loop migration developed by the Connecticut Warbler and several other small warblers. Radar observations have shown that wind directions are more favorable for an east coast fall migration, and the little birds need all the help they can get. Additionally, birds may be following more traditional paths based on their species' historical ranges.

Connecticut Warbler migration.  From the NPWRC.

Whatever the reason loop migrations develop for a species, most DC birders are very excited to see these birds come flocking back through our woods. So far the passerine migration has been pretty quiet while the shorebird migration is in full swing. Stay tuned in the next day or so as I compile a list of and directions to some of DC's most popular fall migration spots, with the help of members of the Va-Bird and MDOsprey listservs.
In DC itself, Rock Creek is still pretty much the place to be for songbirds. The Arboretum's Azalea Garden is very good for thrushes towards the end of September and the beginning of October.
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