AGE STRUCTURE OF NEOTROPICAL MIGRANTS DURING FALL MIGRATION IN SOUTH DAKOTA: IS THE NORTHERN GREAT PLAINS REGION AN INLAND 'COAST'?

摘要

Proportions of juvenile birds in migrant populations at coastal stopover sites are often higher than proportions of juveniles at inland stopover sites, a phenomenon called the "coastal effect." The northern Great Plains forms the western boundary of the migratory ranges for many Neotropical woodland migrants. Moreover, woodland habitat is scarce in this region, consisting primarily of natural riparian corridor woodlands (hereafter corridors) and planted woodlots and shelterbelts around farmsteads and agricultural fields (hereafter woodlots). Due to the scarcity of woodland habitat, one might expect that adults—by virtue of their experience and better-developed navigational abilities—would avoid the Great Plains during fall migration, thus producing age structures in migrant populations similar to those found at coastal sites. We tested this "inland coast" hypothesis for Neotropical migrants captured during fall migration at corridor and woodlot stopover sites in southeastern South Dakota. We classified migrants into three groups based on whether our South Dakota study sites were peripheral (western edge of migratory range within South Dakota), near-peripheral (western edge in, or just west of, Wyoming or Montana), or central (western edge well west of Montana and Wyoming) to their regular migratory ranges. We captured higher juvenile proportions of peripheral (86.5 ± 10.4%, 2 species) and near-peripheral (85.6 ± 4.5%, 7 species) migrants than central migrants (79.6 ± 7.2%, 5 species), but the difference was not significant (P = 0.11). The proportion of juveniles was inversely correlated with the distance of our study sites from the western edge of a given species' migratory range (r = 0.476, P = 0.085). The proportion of juvenile Neotropical migrants at our study sites was near the upper end of proportions recorded for other inland sites in North America, but was slightly lower than those recorded at coastal sites. Our results are, therefore, only partially consistent with the inland coast hypothesis, suggesting that factors other than geographic location also shape age structures at our study sites. The mean proportion of juveniles (for species captured ≥13 times in both habitats; n = 10) was significantly greater (P = 0.002) in the woodlot (89.1 ± 7.0%) than in corridors (73.8 ± 12.5%). The differential age structure in the two habitats suggests that habitat characteristics may influence age structure of the migrant community, but mechanisms for such an influence arc currently unknown.