WITHIN-YEAR 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 mi gration in northeastern North America, We used Hawk Mountain Sanctuary 's 55-year database to calculate abundance indices of 14 raptor specie s at the site. These indices, together with data taken from coincident al U.S. Department of Commerce daily weather maps, were used to invest igate the extent to which raptor migration at Hawk Mountain is associa ted 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 emer ged, which differed in the number of days after frontal passage on whi ch peak migration occurred. The patterns fit the night behavior of the species involved. We failed to detect a long-term linear trend in num bers of fall-season cold fronts at the site. Stepwise multiple-regress ion analyses revealed that annual-abundance indices were not influence d by annual variation in the numbers of cold fronts passing the Sanctu ary at the time of fall migration. Overall, our analyses confirm the r esults of earlier, short-term studies demonstrating within-year effect s of frontal passage on raptor migration. Our failure to detect among- year effects of frontal passage rates on annual counts of raptors migr ating past the site supports the notion that cold fronts enhance fall migration rather than simply making the birds more visible to observer s at such times by forcing them closer to the watch site.