Monday, June 01, 2009

Cold Front Effects on Migrating Raptors

I have continued my reading of research papers as background material for my upcoming research on the related timing of fall migration of avian predators (accipters) and avian prey (songbirds). A couple of papers were duds and not worth writing about. This one however is a very good paper and very relevant to my work.

Paul E. Allen, Laurie J. Goodrich, & Keith L. Bildstein (1996). Within- and Among-Year Effects of Cold Fronts on Migrating Raptors at Hawk Mountain, Pennsylvania, 1934-1991 The Auk, 113 (2), 329-338

The paper looks to answer what effect the passage of cold fronts has on raptors during the fall migration. Using 55 years worth of data from migration monitoring at Hawk Mountain Pennsylvania and weather data from the National Oceanic and Atmospheric Administration, the researchers had ample data to work with. In addition to looking within single years for the correlation, they also used multiple year data to illustrate that the storm fronts were not just making the hawks easier to count.

The research illustrates that the passage of storm fronts has a significant impact on the number of hawks passing by Hawk Mountain. This increase was significant in 13 of the 14 species being monitored with the Rough-legged Hawk (Buteo lagopus) being the only exception. The various classes of raptors behaved differently relative to the storm front, and aligned into three different response groups. The first group (Bald Eagle (Haliaeetus leucocephalus), Ospreys (Pandion haliaetus), Merlins (Falco columbarius), American Kestrels (Falco sparverius), and Peregrine Falcons (Falco peregrinus)) peaked in numbers on the day of the cold front passage. The second group (Golden Eagles (Aquila chrysaetos), Sharp-shinned Hawks (Accipiter striatus), Cooper's Hawks (Accipiter cooperii), and Northern Goshawks (Accipiter gentilis)), which includes my three study species (the accipiters), peaked the day after the storm front passed. The last group, the Buteos (Red-shouldered Hawks (Buteo lineatus), Red-tailed Hawks (Buteo jamaicensis), and Broad-winged Hawks (Buteo platypterus)), peaked 1 to 3 days later. This can be explained by the various flight patterns of the birds involved (altitude, soaring, etc).

To illustrate that the weather patterns were not simply making the hawks more visible to the observers, the researchers compared the number of cold fronts occurring in each migration season (range 10-20) with the total raptors counted each year and found that there was little correlation in the variability. This analysis strengthens the findings that cold fronts do have an effect on the daily migration volumes of raptors.

As with each of these papers I walk away with more questions than answers. The first question is whether the data from the Idaho Bird Observatory agrees with their conclusions. Do weather impacts in the western United States have similar impact to the east? What are the effects on migrating songbirds? I will have to answer these questions to complete my work. At least my three predator species (Sharp-shinned Hawks (Accipiter striatus), Cooper's Hawks (Accipiter cooperii), and Northern Goshawks (Accipiter gentilis)) each reacted consistently to the storm front, this will make it easier than if they each reacted differently.

I will be working this summer at the Idaho Bird Observatory for songbird banding and hawkwatch, both relevant to my research. In addition, I will be utilizing 13 years worth of data collected at the site. I will also spend some time banding raptors and owls.

1 comment:

Anonymous said...

very helpful