Fall Raptor Migration Phenology and its Relationship with Weather

Abstract

Changes in fall raptor migration phenology have been documented at hawk count sites across North America. Delays in fall migration phenology are the most common shift reported, however these changes vary from species to species. Changes in fall migration phenology are often attributed to climate change, but direct links to climate are rarely demonstrated. Those studies that do attempt to link the shifts to climate change often use global weather phenomena such as the North Atlantic Oscillation. Using updated methods, we examined fall migration phenology in 14 raptor species counted in 44 years at Hawk Ridge in Duluth, MN in relation to local and regional climate variables. Variables explored were related to temperature, favorable wind conditions, and cold fronts because these variables have been previously found to influence raptor migration. Weather variables were summarized using a principal components analysis. Raptor phenology was regressed against the top ten principal components using univariate models. Most raptors were found to be migrating later when temperatures were warmer. Favorable winds were also found to influence fall migration timing for most species, however some species responded to a greater percentage of the season having favorable winds, while others responded to greater wind strength of favorable winds. The weather variables used in this study provide a partial explanation for observed changes in migration phenology, but more study is needed to fully explore the mechanisms governing the timing of fall raptor migration.