Migrating birds reorient toward land at dawn over the Great Lakes, USA

Abstract

Most landbirds migrate at night and typically make landfall in suitable stopover habitat before dawn. However, when birds find themselves over large water bodies at dawn, they must continue flying into the day and either finish crossing the water body and land on the far shore or backtrack to the near (i.e. first encountered) shore to land. Their collective decisions will influence how migrants are distributed among shoreline stopover habitats on either side of the water crossing. We studied birds during 4 spring migration seasons from 2010 to 2013 in the Great Lakes region, USA. We used 3 weather surveillance radars to observe migrating landbirds' behavior at dawn and subsequent terrestrial distributions during stopover. Mean flight heights over land and water were higher and mean flight directions were more oriented toward the closest shore at dawn when compared to peak migration earlier in the night. The wider the lake crossing, the higher that birds along the lakeshore flew at dawn. Seasonal mean stopover densities of migrants on land within 3 km of shorelines were 48% higher on the near shores of lakes (based on the seasonal mean direction of migration) than on the far shores. There was a moderate positive correlation (r = 0.584, P < 0.001, n = 358) between the seasonal mean density of birds aloft over water at dawn and the stopover density of birds in adjacent shorelines. Thus, birds over the water at dawn may tend to return to the near shore of the Great Lakes for stopover rather than continuing across the water. As a result, shoreline habitats on the near shores of lakes harbor greater densities of migrants and are thus critical stopover sites for migrating landbirds in the Great Lakes region.