Within- and among-year effects of cold fronts on migrating raptors at Hawk Mountain, Pennsylvania, 1934-1991

Cold-front passage has long been associated with south-bound raptor migration in northeastern North America, We used Hawk Mountain Sanctuary's 55-year database to calculate abundance indices of 14 raptor species at the site. These indices, together with data taken from coincidental U.S. Department of Commerce daily weather maps, were used to investigate the extent to which raptor migration at Hawk Mountain is associated with the passage of cold fronts. Daily abundance indices for 12 of 14 raptor species exhibited significant increases during one or more of the three days following frontal passage. Three basic patterns emerged, which differed in the number of days after frontal passage on which peak migration occurred. The patterns fit the night behavior of the species involved. We failed to detect a long-term linear trend in numbers of fall-season cold fronts at the site. Stepwise multiple-regression analyses revealed that annual-abundance indices were not influenced by annual variation in the numbers of cold fronts passing the Sanctuary at the time of fall migration. Overall, our analyses confirm the results of earlier, short-term studies demonstrating within-year effects of frontal passage on raptor migration. Our failure to detect among-year effects of frontal passage rates on annual counts of raptors migrating past the site supports the notion that cold fronts enhance fall migration rather than simply making the birds more visible to observers at such times by forcing them closer to the watch site.