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Leapfrog migrant
Scientists are revealing secrets about Wilson’s Warbler and other birds by studying atoms locked in their feathers
Contributed by Paul Kerlinger
Published: April 23, 2010 Wilson’s Warbler is so widespread and common that you might think we know all about its migrations. Scientists, however, have uncovered the best-kept secrets of the warbler’s seasonal movements only recently — and only by employing the latest scientific techniques.
The small black-capped warbler nests in riparian areas or wet meadows near deciduous shrub thickets from Newfoundland and New England west to Alaska and south to California and the Rockies. In winter, the species occurs in a variety of habitats from the Pacific coast of Mexico to the Gulf and Caribbean coasts from southern Louisiana and Texas to Panama.
Over the last decade, several biologists who are curious about the warbler have opted not to rely solely on visual observations, radar, acoustical monitoring, thermal imaging, banding, and other standard migration-study methods. Instead, they’re also turning their attention to the new field of biogeochemistry, the study of earth chemicals that are incorporated into and interact with all living creatures. Specifically, they’re examining atoms known as stable isotopes, which are found naturally in the feathers of all birds.
Stable isotopes of hydrogen, nitrogen, carbon, and other elements do not decay over time, making them useful to migration biologists. The quantity of isotopes in the environment varies predictably across the earth by temperature, annual precipitation, and latitude. When a bird grows new feathers, the isotopic signature unique to the region in which the feathers are grown remains in its feather chemistry. By studying the feathers, scientists can get a good idea of where the bird was when it grew them.
Researchers have learned that the ratio of the stable isotopes of hydrogen in the feathers of Wilson’s Warblers varies by latitude. Jeffrey Kelly, an associate professor of zoology at the University of Oklahoma, studied the primary and body feathers of museum specimens and migrants. He and colleagues at the Rocky Mountain Research Station in New Mexico found that among warblers from the western United States and Canada, individuals that nested farthest north migrated to the most southerly portions of the wintering range.
In other words, warblers from Alaska and the Yukon Territory were more likely to migrate to Costa Rica and Panama. Birds that nested in California or Colorado migrated only as far as northern or central Mexico. The northern birds leapfrogged over birds that nested farther south.
Northern birds go first
By studying birds’ arrival times at Bosque del Apache National Wildlife Refuge in New Mexico, Kelly and his team also found that the warblers that bred farthest north migrated earlier in fall than southern breeding birds. The pattern was consistent among adult males, hatch-year males, and hatch-year females. (The sample size of adult females was not large enough for a robust statistical analysis.)
Kristina Paxton, a Ph.D. student at the University of Southern Mississippi, and her colleagues from the U.S. Geological Survey found further evidence of a leapfrog migration. She examined stable isotopes in feathers of Wilson’s Warblers captured at five banding stations across the southwestern U.S. and found that migrants from the southern breeding populations migrated earlier in spring. Individuals that had nested in the northern part of the range migrated later and leapfrogged over the more southern breeding birds after they were already on territories.
The usefulness of studying stable isotopes goes far beyond the example of the Wilson’s Warbler. The list of birds for which scientists have examined such “flying fingerprints” includes the Red-winged Blackbird, Loggerhead Shrike, White-winged and Mourning Doves, Cooper’s Hawk, White-crowned Sparrow, American Redstart, and Black-throated Blue Warbler.
Although stable-isotope analysis cannot pinpoint the exact nesting areas of birds, other research methods may be used to fine-tune our understanding of migration. For example, museum specimens provide a massive amount of material from which to examine variation in migration of species, especially when combined with plumage traits, banding, and visual studies.
I’m fascinated by the migration stories we’ve learned from the feathers of Wilson’s Warblers and other birds, but I’m equally amazed by the people who study migration. Once a question is posed, migration biologists doggedly pursue answers, finding new ways to learn about migration. To the traditional research methods — sight records, banding, radar, acoustical sensing, and thermal imaging — they have added stable-isotope analysis. So what’s next?
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Migration glossary
Stable isotopes: Atoms of an element with equal numbers of protons and neutrons. They are not radioactive and do not decay.
Biogeochemistry: The study of earth chemicals that are incorporated into and interact with living entities. |
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