Age ratios and their possible use in determining autumn routes of passerine migrants

Abstract

Age ratios and abundance of 61 migrant passerine species comprising more than 42,000 birds were analyzed in an attempt to determine patterns of migration. The data were collected at 10 stations from coastal Massachusetts to inland Pennsylvania. Age ratios are thought to be useful in determining routes of migration. The principal criteria for the edge of a species' route are suggested to be a higher proportion of young and a lower density of the species than in other areas. Almost all species in this area fell readily into 1 of 5 patterns that suggest 5 possible routes: (1) immediately along the coast (3 species); (2) on the coastal plain (24 species); (3) west of the Appalachians (17 species); (4) overwater, direct to South America (only the Blackpoll Warbler); and (5) an unconfined, broad front, encompassing the entire area (14 species). For 3 species, no route could be determined. Almost all species showed the coastal effect, a higher percentage of young along the coast than elsewhere. By the criteria given, this indicates that the coast is the edge of the migratory route of most species. Most probably the young found near the coast lack some navigational capabilities and are off course; many of them probably perish. In general, a given route was not strongly associated with either diurnal or nocturnal migration, distance to wintering grounds, or with any genus or family of birds. I speculate that this is evidence that routes have evolved independently in each species.

Citation

Ralph, C. John. 1981. Age ratios and their possible use in determining autumn routes of passerine migrants. Wilson Bulletin 93(2):164-188