Within- and Among-Year Effects of Cold Fronts on Migrating Raptors at Hawk Mountain, Pennsylvania, 1934-1991

Abstract

ABsTRAcr.-Cold-front passage has long been associated with southbound 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 flight 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 multipleregression 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 withinyear 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.

Effects of Regional Cold Fronts and Localized Weather Phenomena on Autumn Migration of Raptors and Landbirds in Southwest Idaho

“…Richardson (1978) and Allen et al (1996) found migration rates to peak the day after a cold front's passage. We believe differences in the frequency of cold fronts to be the primary factor behind this discrepancy.…”

“…Following Millsap and Zook (1983), Hall et al (1992), and Allen et al (1996), we determined passages of cold fronts by analyzing synoptic weather maps obtained from the National Oceanic and Atmospheric Administration's (NOAA) Daily Weather Map series (U.S. Department of Commerce 2010a). We interpolated the timing of each front through examination of subsequent daily maps.…”

“…Fronts that appeared or disappeared on subsequent maps, with no evidence that they had passed the study site, were not included in analyses. We counted stationary fronts on the last day they remained over the study site (Hall et al 1992, Allen et al 1996. For the purposes of modeling, we classified days as "under the influence of a cold front" if they fell the day before, the day of, or the day after passage of a cold front.…”

“…However, is the effect of weather on migration similar across diverse geographic regions and across all species? Many site-specific studies (Titus and Mosher 1982, Millsap and Zook 1983, Hall et al 1992, Allen et al 1996, Woltmann and Cimprich 2003 and syntheses (Richardson 1978(Richardson , 1990) document our current understanding with regard to weather and avian migration. With respect to cold fronts, some studies have shown peak migration of raptors on the day a front passes (Millsap and Zook 1983), on the day after it passes (Richardson 1978, Allen et al 1996, or no significant relationship at all (Hall et al 1992).…”

“…Many site-specific studies (Titus and Mosher 1982, Millsap and Zook 1983, Hall et al 1992, Allen et al 1996, Woltmann and Cimprich 2003 and syntheses (Richardson 1978(Richardson , 1990) document our current understanding with regard to weather and avian migration. With respect to cold fronts, some studies have shown peak migration of raptors on the day a front passes (Millsap and Zook 1983), on the day after it passes (Richardson 1978, Allen et al 1996, or no significant relationship at all (Hall et al 1992). Among local weather variables, some studies have shown only wind direction and visibility factors as significant predictors for some species (Titus and Mosher 1982), while others have found a much more diverse set of significant predictors such as wind direction and speed, temperature, pressure, cloud cover, and lunar phase (Pyle et al 1993).…”