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Geo-Locaters Yield Data about Songbird Migration

March 25, 2009 - Keeping track of bird migration is a time-honored practice of naturalists dating back more than 200 years.

Until recently, our understanding of the timing of bird migration was based on populations of birds rather than individuals. We have to be able to follow individual birds to truly understand migration speeds and routes.

Bird banding has contributed some useful information. However, the chance of a bird bander capturing a bird banded earlier in the migration is pretty slim. The chance of capturing a banded bird immediately after its arrival is slimmer yet.

Outfitting birds with radio-tags that emit unique frequencies provides a way to track individual birds. The range of most radio-tags is limited to a couple of miles, so this technology is more useful for tracking the movements of resident birds.

More recently, satellite transmitters have been developed that allow ornithologists to monitor the position of birds from their computers. For instance, researchers used satellite transmitters to track the migration of short-tailed albatrosses across the Pacific.

Although radio and satellite transmitters have been miniaturized, they are still too large to place on most songbirds. A new technology, called geo- locating, promises to provide new insight into the pace and direction of small bird migration. Songbird geo-locators are essentially a small computer chip with a built-in clock and a short stalk.

The geo-locater tracks light levels so that sunrise and sunset are recorded for each day. Knowing sunrise, sunset and day length allows researchers to determine the position of the bird every day.

Essentially, the length of the day allows the latitude to be determined, and the time of sunrise and sunset allows the longitude to be determined.

The first results of this technique recently were published by a team of researchers from York University. The team captured 20 purple martins and 14 wood thrushes in the fall of 2007 in northern Pennsylvania. They were able to recapture five of the wood thrushes and two of the purple martins in the spring of 2008. The researchers downloaded the data from the geo-locater and were able to map the fall migration, winter movements and spring migration of each bird.

Even though only seven birds were recaptured, the results already cause us to rethink how migration occurs in songbirds. In the fall, the two purple martins flew south to the Yucatan Peninsula in five days (about 1,500 miles in total). The martins stopped there for three to four weeks before continuing their migration to a wintering area in Brazil.

We know that spring migration is typically much faster than fall migration in most birds. However, the speed of the purple martin spring migration was more rapid than suspected.

One of the tagged martins flew from the Amazon Basin back to Pennsylvania in only 13 days, traversing more than 3,000 miles. Four of those days were spent on stopover.

Prior to this work, ornithologists believed that a nightly flight of about 100 miles was about the maximum distance most songbirds could manage.

Four of the five wood thrushes migrated to the southeastern United States, spending one to two weeks there before flying across the Gulf of Mexico to the Yucatan Peninsula. The birds wintered in Honduras and Nicaragua.

Most of the thrushes returned to Pennsylvania in 13 to 15 days, again crossing the Gulf of Mexico. One thrush took the landward route, avoiding the Gulf crossing.

HERB WILSON, Portland Press Herald, March 19, 2009


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