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For immediate release: June 17, 2003
Contact: Steven Schultz, (609) 258-5729, sschultz@princeton.edu

A bird in the bush is worth … a lot
Study of energy costs in birds finds that resting places are important during migration

PRINCETON, N.J. -- Highway travelers know the relief of finding a great place to stay after a long night on the road. Migrating birds are no different. A study led by a Princeton researcher has shown that birds expend more energy seeking food and shelter between the legs of their journey than they do flying.

"Flight itself is costly, but foraging and preparing for the flight is even more costly," said Martin Wikelski, a biologist and lead author of a study that involved tracking birds for hundreds of miles as they traversed the American Midwest.

One implication of the finding is that urban and suburban homeowners play a critical role in preserving migratory routes. The birds followed in the study often stopped in the backyards of populated areas to look for food and prepare for the next flight. "You really have to keep your garden in good shape so the birds can come through and make it up to Canada," Wikelski said. Migrating birds need bushes and some natural undergrowth, not just lawn and flower gardens, he said.

The researchers conducted the study by trapping migrating thrushes in Urbana, Ill., and injecting them with water that contained harmless heavy isotopes of hydrogen and oxygen. Working with one bird at a time, they attached a small radio transmitter between its wings and released it. They then drove through the night tracking the bird until it landed at dawn, covering an average of 170 miles over 4.6 hours. One flight went 370 miles and lasted 7.7 hours. With the permission of the owner of the home where the bird landed, they recaptured the bird, took a blood sample, weighed it and measured its body fat. The concentration of the heavy isotopes remaining in the bird provided a measure of how much energy it had expended.

The researchers followed 12 birds for one night, but could only recapture and blood sample six of them at their new destination. They compared their measurements to those of other birds that did not migrate. Reporting the results in the June 12 issue of Nature, Wikelski and colleagues concluded that actual flight accounts for only 29 percent of the energy birds expend during their total migration, which includes days when the birds do not fly.

Although other researchers had previously theorized that stopovers might consume more energy than flying, the study is the first to make actual measurements in the field and coordinate them with weather, habitat and other conditions. One reason for the relatively low cost of flight may be that the exertion helps keep birds warm at night; while resting they have to expend a lot of energy just to keep warm, said Wikelski.

In addition to the specific findings, said Wikelski, the study demonstrates the potential for rigorous, quantitative field research on birds. Previous research focused on studying bird migration in the lab or looking at general patterns of weather and migration, but did not track individual songbirds in the field. Although radio tracking is challenging, it offers a chance to pull together various areas of research in a real-life situation, he said.

Wikelski and colleagues are now conducting further studies in which they continuously monitor the heart rates of birds during migration and look more closely at the effects of weather and atmospheric dynamics.

Wikelski, who came to Princeton's Department of Ecology and Evolutionary Biology in 2000 from the University of Illinois at Urbana-Champaign, conducted the study in collaboration with colleagues at that school and with researchers from the Illinois Natural History Survey, Groningen University in The Netherlands and the Zoological Laboratory of Haren, The Netherlands.

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