Radar Analysis of Fall Migration Stopover Sites in the Northeastern U.S.

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The Challenge: Most landbird conservation efforts focus on protecting or enhancing breeding habitat. For migratory species, however, mortality is highest during the biannual migration periods. In fall, juvenile birds are making their first migratory flights; their success, and that of adult birds, depends on availability of suitable sites to safely rest and forage. Identifying important stopover sites and habitats is thus a critical step to develop comprehensive conservation plans for migratory landbirds. We used weather surveillance radar data from Fall 2008–2014 to model and map the spatial distribution of landbird migrants at stopover sites across the northeastern U.S.

The Science: Radars detect landbirds as they leave stopover sites to embark on nocturnal migratory flight. We georeferenced the aerial density of birds, as measured by radar reflectivity, to their ground sources. Radar-observed stopover densities of migrants declined by 29% over the seven years studied. Overall, predicted densities increased closer to bright areas and the Atlantic coast, and with forest cover at multiple scales. In field surveys, migrant densities were related to food resources and shrub density in forests, forest cover in the landscape, and geographic location. Integrating field and radar data, we classified surveyed sites as to their ecological function. Most were ‘fire escapes’ that offer a resting place for migrants but have limited food resources.

The Future: Maps of predicted stopover densities identify sites with high or consistent use. Migratory flights could be simulated or tracked to see if portions of the region with predicted low use are bypassed due to their relative location or because few suitable stopover sites exist. Results can focus conservation efforts for migrating landbirds on areas where they likely will be most effective. The maps also can serve as a sampling frame to design field studies of migrants. More research is needed to verify model predictions and to better understand how migrants use sites in order to further prioritize sites for conservation and to identify habitat enhancement techniques that benefit migrants.