Saturday, May 09, 2009

Differential Autumn Migration of Accipiter Hawks

I have been narrowing in on the specification of my undergraduate research project in Biology/Ecology. The current proposal is related to correlation of migration timing between Predators (Accipiter Hawks) and Prey (Songbirds) at the Idaho Bird Observatory. In the process of refining my research objectives and proposals, I have begun the review of relevant prior research. This is the first paper I have reviewed.

The two most relevant predator species for songbirds are the Sharp-shinned Hawk (SSHA) and the Cooper's Hawk (COHA). The chosen article analyzes the timing of migration between Sharp-shinned Hawks and Cooper's Hawk, differences between male and female migration timing, differences between juveniles and adults, and the differences from year to year.

John DeLong, & Stephen W. Hoffman (1999). Differential Autumn Migration of Sharp-Shinned and Cooper's Hawks in Western North America The Condor, 101 (3), 674-678

Its worth noting that one of the study sites referenced in this paper is likely on the migration flyway south of the Idaho Bird Observatory. Thus, the populations being monitored at that site is likely part of the same migratory population that I will be studying.

The authors reviewed data from two hawk banding sites, one in New Mexico and one in Nevada, over a five year period (1992-1996). They only used banding data due to the unreliable sexing of non-captured birds. Banded birds are captured, where non-captured birds are only observed flying over the monitoring site. Most of the results analyzed are statistically significant, with a couple of non-significant points dealing with the year to year comparisons at one of the monitoring sites.

The authors discovered the general migration order to be juvenile females, juvenile males, adult females, then adult males. The Sharp-shinned Hawks had greater spread in these classes than the Cooper's Hawks. The Sharp-shinned Hawks on average came through the monitoring sites later in the migration season than the Cooper's Hawks.

There are a number of possible explanations for these results provided in the discussion section of the paper. The main question is whether the birds leave their breeding area at the same time. Most likely the juveniles leave early. Whether this is based on their decreasing ability to capture food, or for some other reason is not known. The sex differences could be related to the females leaving the breeding site earlier or the females simply migrating at a faster rate. Since the females are larger than males (reverse sexual dimorphism), they fly faster and have greater endurance. This could be the reason for the differential timing at these southern monitoring stations. It is also possible that the females leave early as their prey base may be less available than the food base for the smaller males. A similar explanation could be used for the differences between the two species. The smaller species (Sharp-shinned Hawk), could be flying slower or could have stayed longer due to different prey base availability. We just don't know. It would be interesting to know if the differential timing is the same at more northern monitoring stations like the Idaho Bird observatory. That could be a whole study in itself.

The results of this paper clearly complicate my potential project. I now must consider the predator species (SSHA or COHA), male/female, and adult/juvenile populations differently. It's definitely a fascinating area of research. I will post additional article reviews as I read them.

, , , ,


John B. said...

Accipiters at Cape May follow a similar order by age and to a lesser extent by sex. I'm inclined to think that the immatures go first because of their incomplete hunting skills. It seems like it would be easier to find food by following songbird flocks. It would be interesting to see some confirmation for that.

wolf21m said...

John, Thanks for the Cape May experience.

I think I will first look at what our 13 years of data at Idaho Bird Observatory show. I know that it too shows juveniles first, but not sure about sex. As to why juveniles travel first, it appears that the leading hypothesis is their inability to get food in the breeding territory. If my research shows the correlation with songbirds, we might be on to something. Do you think anyone at Cape May would like to see my preliminary finding when I have some (this fall)?

I look forward to working at the IBO. I have observed songbird banding, hawk banding, hawk watch, and owl banding, but have not participated directly myself. This summer will be my first hands on.