Determination of raptor migratory patterns over a large landscape

Each autumn, tens of thousands of raptors pass over Hawk Ridge in Duluth, Minnesota, on their southbound migration, but migratory pathways leading to Hawk Ridge are unknown. To address this issue, we counted migrating raptors between mid-August and mid-November 2008 from 24 observation points along eight transects perpendicular to the shoreline between Duluth and the Minnesota-Canadian border. Our goals were to determine migratory pathways over a large area (>2000 km ²) and identify how these movements were affected by weather, time of day, season, and characteristics of the landscape. A total of 4303 raptors of 14 different species were counted during the 2008 migration season. Exploratory analyses suggested that migratory raptors concentrated near the northern shoreline of Lake Superior, particularly during midday when winds are westerly. Average migration height differed between soaring raptors (buteos and eagles) and accipiters, with >40 of soaring raptors observed higher than 100 m above the tree canopy and ≥30 of accipiters observed lower than 100 m above the tree canopy. Mixed models analysis identified the significant factors (P < 0.05) associated with total raptor migration: wind direction, time of day, temperature, and antecedent wind (number of days in which the wind did not have a westerly component prior to the observation days; R ² 0.23). Significant factors associated with soaring-raptor migration included wind direction, time of day, temperature, and seasonal interval (e.g., early or late in the migration season; R ² 0.17) and those associated with accipiter migration included time of day, temperature, antecedent wind, wind direction, and seasonal interval (R ² 0.29). With the increasing popularity of wind power development, information is needed regarding avian migratory pathways to avoid bird-turbine conflicts. The methodology and design of this study provided a means to quantify the magnitude, timing, pathways, and weather conditions associated with raptor migration over a large landscape.