POPULATION TRENDS OF MIGRATORY LANDBIRDS ALONG THE MIDDLE RIO-GRANDE

Using standardized mist-netting capture data from the Rio Grande Natur e Center, Albuquerque, New Mexico from 1985-1994, and Breeding Bird Su rvey data from New Mexico and United States from 1980-1994, we compare d local, regional, and national population trends of landbird species that migrate along the middle Rio Grande in fall. Evaluations of relat ionships among population trends, migratory distance, breeding habitat use, nest type, and foraging guilds revealed that population trends o f Rio Grande migrants were relatively species-specific and that popula tion changes were unequally distributed among habitats and life histor y groups. The fall banding data suggested that long-distance migrants, riparian forest migrants, canopy insectivores, and open-cup nesting s pecies declined more over the 10-year period than other groups. Simila rities between population trends from banding data and trends from Bre eding Bird Survey increased as we expanded the geographic coverage of the Breeding Bird Survey data from New Mexico to the entire United Sta tes. We hypothesize that riparian habitat along the middle Rio Grande functions as a funnel that constricts habitat use during migration for species whose breeding and wintering populations are spread over broa der geographic areas and that local population changes detected during mass migration may thus reflect widespread and large-scale changes.