Thursday, May 14, 2009

Weather effects on fall hawk migration

I have continued the background discovery process for my research project on songbird and accipiter hawk (hawks that feed on songbirds) migration patterns. A week ago I reviewed a paper on the differential timing of accipiter hawk migration. That particular paper provided a lot to think about and indicated that I may need a more complicated algorithm for my research. This week I found some time to review a paper on weather impacts on hawk migration. I am interested in the research as I will likely need to factor weather patterns as a variable in my analysis.

Kimberly Titus, & James A. Mosher (1982). The Influence of Seasonality and Selected Weather Variables on Autumn Migration of Three Species of Hawks through the Central Appalachians The Wilson Bulletin, 94 (2), 176-184

The paper focuses on the three hawk species migrating through the central Appalachian mountains. Only one of the three species is an accipiter (Sharp-shinned Hawk - Accipiter striatus), while the other two are Buteos (Red-tailed Hawk - Buteo jamaicensis, Broad-winged Hawk - Buteo platypterus), generally not predators of songbirds. The monitoring sites were located in western Maryland, with total migrating birds compared against Hawk Mountain in Pennsylvania. A number of weather variables were used including wind direction, wind speed, horizontal visibility, cloud cover and temperature. The study was conducted over a five year period - 1975-1979.

The paper found that the number one variable in the analysis for predicting migration was the date, followed by the time of day. These were far more significant than any other factors. In other words, when a bird begins a migration, it most likely just copes with the weather conditions as they are dealt. This is not a surprising find in my mind. Further, for the species of my interest, the Sharp-shinned Hawk - A. striatus, they found no significant interaction between wind speed or direction, although a general correlation of good visibility and tail winds was present. The Broad-winged Hawk - B. platypterus, had similar results with a general correlation with good visibility and strong favorable winds. Only the Red-tailed Hawk - B. jamaicensis, produced a significant result. They tended to favor light, opposing winds from the Southeast. These results conflict other findings cited in the paper.

The results seem somewhat logical to me. The Sharp-shinned Hawk spends significantly more time providing its other thrust, than the Buteos which are more dependent upon thermals and gliding. The thermals would be weather dependent, thus you would expect a more significant result for these birds. These results could simplify my work, although they raise more questions than answers. I do not believe this study is sufficient for me to remove weather from my model. I will have to study the other references cited.


Anonymous said...

That's interesting some hawks favor the flavor of a song bird.

I'm curious about something, though, when you say "reviewed a paper", do you mean you "peer-reviewed a paper" or that you "read a paper"? I hope its the latter, not the former. Blogging a paper you reviewed would be waaay bad, dude.

wolf21m said...

Anonymous, I agree that blogging a paper that I peer reviewed would be inappropriate. This is simply a "peer reviewed" paper that I have read. I was not a peer reviewer.

Anonymous said...

just checking, one science blogger to another. thanks for the great posts.

John said...

That time of year and day would be the most important variables makes sense, but I'm not sure that I entirely buy a lack of correlation between weather conditions and migration. It would be interesting to see how weather effects compare between sites.