Molt Strategies and Age Differences in Migration Timing among Autumn Landbird Migrants in Southwestern Idaho
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چکیده
A .—Intraspecifi c pa erns of autumn migration timing are not well known, particularly in the western United States. Here, we (1) describe autumn migration timing and age ratios of landbird migrants in southwestern Idaho, (2) examine differences in timing among age and sex classes, and (3) demonstrate how prebasic molt strategies aff ect migration timing diff erences between age classes. As a group, Neotropical migrants were most common from late July through early September, whereas temperate migrants were most common from mid-September into early October. Proportion of hatch-year birds was 74.5% for all migrants combined and ranged from 33.3% to 100% for individual species. Timing diff erences between sex classes were detected in only a few species and no general pa erns emerged. In 22 of 31 Neotropical and temperate migrants examined, there were signifi cant diff erences in timing between adults and hatch-year birds. In species in which adults begin fall migration before replacing fl ight feathers, adults migrated earlier than hatch-year birds. Conversely, in species in which adults molt fl ight feathers on or near the breeding grounds before departing on fall migration, hatch-year birds migrated earlier than adults in all but one case. Therefore, it appears that molt strategy is a powerful determinant of intraspecifi c migration timing diff erences and, to our knowledge, this is the fi rst study to document this pa ern among migrant passerines of North America. Received 21 February 2004, accepted 11 March 2005.
منابع مشابه
Molt Strategies and Age Differences in Migration Timing Among Autumn Landbird Migrants in Southwestern Idaho
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