The Influence of Topography, Wind, and Time of Night on Migratory Songbird Orientation along the Southwest Coast of Lake Erie
Many long-distance migratory songbirds encounter geographic barriers at some point during their annual migrations. Bird migration routes often follow detours where passages across ecological barriers are either avoided entirely or reduced in extent. Numerous early reports of broad front migration crossing coastlines seemingly without taking notice of the transition from land to water contradict more recent observations suggesting important deviations from the standard broad front direction associated with the underlying topography, in particular, the course of coastlines. However, whether and under what conditions nocturnally migrating songbirds would respond to a more modest obstacle such the Great Lakes is less well understood. I used a combination of weather surveillance radar and thermal imaging cameras to capture the direction of spring migration both regionally, as migrants are approaching the southern shore of Lake Erie, and locally along the coast. In the present body of work, I have studied (1) the nocturnal migratory orientation of birds along the southwest coast of Lake Erie during the spring migratory season, (2) the orientation of migrants with respect to the prevailing winds, and (3) the orientation with respect to time of night, which presumably is reflective of differences in the energetic condition of migrants. Along the southwest coast of Lake Erie, the direction of migration differed significantly from the broad front direction approaching Lake Erie, and the probability of a migrant deviating from the broad front direction was higher at sites which presented southeast-northwest oriented coastlines (Cedar Point and Ottawa NWR). Even under energetically favorable winds above Cedar Point and Ottawa some migrants were observed detouring the Lake, and migration in crosswinds increased the likelihood of either coastal flights in easterly winds or crossing flights in westerly winds. Migrants approaching the shore above Ottawa showed a significant increase in the percent of flight bearings deviating along the coast late in the night compared to early in the night, possibly reflective of a decline in motivation to cross as energetic stores become depleted during the night. Migrants approaching the southern shore above Maumee Bay, an east-west oriented coastline, were more consistent with observations made inland, away from any presumptive effects of the coast, and made a Lake crossing regardless of winds aloft or time of night. As a whole, the present dissertation expands the scientific understanding of the phenomenon of coastal migration in the context of an understudied system, the Great Lakes, and in particular Lake Erie.