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ScienceWeek
ECOLOGY: MIGRATORY BIRDS AND MOLTING LOCATION
The following points are made by Geoffrey E. Hill (Science 2004 306:2201):
1) For the past 50 years, most field ornithologists studying migratory birds that breed in North America and Europe have concentrated on analyzing their breeding ecology. Molt and migration have received scant attention, as has the time these migratory birds spend at their wintering grounds, even though this period may occupy 8 months of a bird's annual cycle [1]. It is easy to understand why ornithologists, most of whom live in the US, Canada, and Europe, have concentrated on the breeding biology of migratory birds. Not only are there many interesting questions that can be addressed through breeding studies, but also migratory birds are accessible in the spring and summer, are predictable in their movements, and conspicuous in song. After breeding, territories are abandoned, home ranges expand greatly, and birds begin their nocturnal southward movement, making it impossible to track individuals of most species.
2) However, new technologies are helping ornithologists to overcome these obstacles, bringing studies of molt and migration to the fore. Norris et al[2] analyzed the ratio of stable-hydrogen isotopes in the feathers [3] of individual American redstarts (Setophaga ruticilla) of known reproductive history to determine when and where molting takes place.
3) Redstarts are typical Neotropical migratory birds that breed in the eastern US and Canada and winter in Central America and the Caribbean. Norris et al[2] observed that the stable-hydrogen isotope signature of feathers grown while male birds reside at their breeding grounds in Ontario differs from that of feathers grown by birds on the migratory pathway much farther south. Thus, by analyzing feathers for their isotope content, Norris et al[2] could tell the latitude at which individual redstarts molted. They found that some redstarts completed their fall molt on the breeding grounds after they had finished nesting, thereby temporally separating the three most energetically costly activities of the year: breeding, molt, and migration. Other redstarts molted as they migrated.
4) Of most significance is that these investigators found evidence for a trade-off between energy investment in current reproduction, timing of the molt (a critical aspect of self-maintenance), and sexual signaling by males through feather color (a key determinant of future reproduction). Male redstarts that invested more energy in reproduction, including breeding later into the summer, completed their molt farther south on the migratory route, with greater overlap of molt and migration, and these males grew less colorful nuptial plumage. Analysis of stable-hydrogen isotopes in feathers is the only technique by which this striking pattern could have been revealed -- there is essentially no chance of finding and accessing individual study birds during migration, even with radio telemetry.[4,5]
References (abridged):
1. J. Terborgh, Where Have All the Birds Gone? Essays on the Biology and Conservation of Birds That Migrate to the American Tropics (Princeton Univ. Press, Princeton, NJ, 1989)
2. D. R. Norris, P. P. Marra, R. Montgomerie, T. K. Kyser, L. M. Ratcliffe, Science 306, 2249 (2004)
3. K. A. Hobson, L. I. Wassenaar, Oecologia 120, 312 (1999)
4. G. E. Hill, A Red Bird in a Brown Bag: The Function and Evolution of Ornamental Plumage Coloration in the House Finch (Oxford Univ. Press, New York, 2002)
5. D. E. Loria, F. R. Moore, Behav. Ecol. 1, 24 (1990).
Science http://www.sciencemag.org
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NATURAL HISTORY: ON THE MAGNETIC COMPASS OF SONGBIRDS
The following points are made by W.W. Cochran et al (Science 2004 304:405):
1) Billions of songbirds migrate between continents twice each year, but their orientation capabilities are almost exclusively studied in the laboratory. The authors presented birds with experimentally altered orientation cues and followed their subsequent migratory flights in the wild. Avian navigation capabilities are very precise (1), with many individuals returning to the same breeding sites year after year (1-3) after a voyage of up to 25,000 km (4, ).
2) Migratory songbirds can orient on the basis of compass information from the sun and its associated polarized light patterns, the stars, the earth's magnetic field, and the memorization of spatial cues en route. However, the interactions and relative importance of these cues remain unclear and a source of much debate. Our knowledge about the orientation mechanisms of songbirds relies almost exclusively on data from cue-manipulated captive migrants tested in various orientation cages, on vanishing bearings based on the first few hundred meters of flight, and to a much lesser degree on field data (ringing and radar and visual observations) from unmanipulated natural migrants.
3) On clear evenings, the authors fitted Catharus thrushes with radio transmitters and placed them in outdoor cages in an artificial eastward-turned magnetic field from about sunset until the sun was 11 deg or more below the horizon when they were set free. The authors then radio-tracked the birds in flight to obtain heading data. Because Catharus thrushes do not compensate for wind drift but individuals maintain nearly constant preferred headings from night to night, the authors used measured headings for orientation analyses.
4) In summary: Night migratory songbirds can use stars, sun, geomagnetic field, and polarized light for orientation when tested in captivity. The authors studied the interaction of magnetic, stellar, and twilight orientation cues in free-flying songbirds. The authors exposed Catharus thrushes to eastward-turned magnetic fields during the twilight period before takeoff and then followed them for up to 1100 kilometers. Instead of heading north, experimental birds flew westward. On subsequent nights, the same individuals migrated northward again. The authors suggest that birds orient with a magnetic compass calibrated daily from twilight cues, and that this could explain how birds cross the magnetic equator and deal with declination.
References (abridged):
1. P. Berthold, E. Gwinner, E. Sonnenschein, Eds., Avian Migration (Springer, Berlin, 2003)
2. J. P. Hoover, Ecology 84, 416 (2003)
3. P. O. Dunn, D. W. Winkler, Proc. R. Soc. London Ser. B. 266, 2487 (1999)
4. D. C. Outlaw, et al., Auk 120, 299 (2003)
5. W. L. Engels, Biol. Bull. 123, 94 (1962)
Science http://www.sciencemag.org
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ON FUEL LOADS AND BIRD MIGRATION
The following points are made by A. Kvist et al (Nature 2001 413:730):
1) Aerodynamic theory predicts that to support the weight of the bird and to overcome the drag of the body and wings, the mechanical power output the flight muscles generate increases with fuel load. The available evidence indicates that the aerodynamic models correctly describe the essential physical processes involved.
2) Birds on migration alternate between consuming fuel stores during flight and accumulating fuel stores during stopovers. The optimal timing and length of flights and stopovers for successful migration depend heavily on the extra metabolic power input (fuel use) required to carry the fuel stores during flight. The effect of large fuel loads on metabolic power input has never been empirically determined.
3) The authors report they measured the total metabolic power input of a long-distance migrant, the red knot (Calidris canutus), flying for 6 to 10 hours in a wind tunnel. The authors report that total metabolic power input increased with fuel load, but proportionally less than the predicted mechanical power output from the flight muscles. The authors suggest the most likely explanation is that the efficiency with which metabolic power input is converted into mechanical output by the flight muscles increases with fuel load. This will influence current models of bird flight and bird migration, and may also help to explain why some shorebirds, despite the high metabolic power input required to fly, routinely make nonstop flights of 4000 kilometers or longer.
Nature http://www.nature.com/nature
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