Determination of raptor migratory pathways over a large landscape

Abstract

Each autumn, tens of thousands of raptors pass over Hawk Ridge in Duluth, Minnesota on their southbound migration. Although these numbers indicate that the North Shore of Lake Superior is an important migratory corridor, migratory raptor concentrations along the North Shore beyond Hawk Ridge are unknown. To address this issue, migratory raptor counts were conducted from 24 observation sites located between Knife River and Grand Portage, Minnesota, from mid-August through mid-November 2008. My primary objective is to develop a methodology and experimental design to determine the effects of weather, temporal, and landscape factors on raptor migratory pathways over a large landscape. A total of 4,303 raptors were counted migrating through the region among 14 species. Exploratory analyses suggest migratory raptors concentrate near the shoreline of Lake Superior, particularly midday when winds have a westerly component. Flight height differed between Buteos and Accipiters, with >40% of Buteos observed beyond 100m above the tree canopy and approximately ≥30% Accipiters observed below 100m above the tree canopy. Significant factors (p<0.05) were identified using multiple regression analysis for total raptors combined (wind direction, hour, temperature, and antecedent wind; r2=0.28, n=564), Buteos (wind direction, hour, and temperature; r2=0.29, n=564), and Accipiters (hour, temperature, antecedent wind, and wind direction nearly significant with p=0.06; r2=0.28, n=564). With the increasing popularity of wind power, the cumulative impacts on birds are of immediate conservation concern, and it is vital that migratory pathways be identified in detail over large regions to avoid large scale negative effects on migrating birds.