Chapter 1: The Modulation of the Adrenocortical Stress Response in North American Accipiters During Fall Migration
Birds must build fat stores through increased foraging and food consumption to ensure a successful migration. Such hyperphagy is facilitated by an increase of corticosterone secreted from the adrenal glands. To prevent catabolism of flight muscles during migration, birds must modulate their normal stress responses to avoid additional corticosterone release. I tested three species of Accipiter hawks (Sharp-shinned Hawks (Accipiter striatws), Coopers Hawks (A. cooperi), and Northern Goshawks (A. gentilis)) captured during fall migration in southern Idaho for modulation of their stress responses. Each hawk underwent a standard stress protocol with blood taken at 0 mm and 30 mm after capture. Blood was analyzed for corticosterone levels using radioimmunoassay techniques. No significant difference in corticosterone levels between times 0 mill and 30 mm was seen, indicating that Accipiter hawks do not significantly increase corticosterone secretion when stressed during migration. I found no significant difference in baseline corticosterone between males and females in Sharp-shinned Hawks and Northern Goshawks, but a significant difference was found between the sexes in Coopers Hawks. In all three species, there was no significant difference in baseline corticosterone between hatch year and after-hatch year hawks. My results support the Migration Modulation Hypothesis and indicate that Accipiter hawks, like other birds, can dampen their stress responses while on migration, probably to prevent catabolism of their flight muscles when fat stores have been depleted.
Chapter 2: Corticosterone Secretion in Relation to Flight Strategies and Energy Expenditure in North American Accipiters During Fall Migration
Migration is a vital part of the yearly cycle for many birds and is classified as the seasonal movement between breeding and non-breeding areas. In preparation for migration, birds increase the levels or corticosterone secreted by their adrenal glands. This not only prepares them for migration, but also aids in ensuring that there is adequate energy (glucose) available for long distance flight. There are a variety of factors that affect the baseline level of corticosterone in birds. These can include size, flight behavior, energetics, and migratory status. I analyzed three species of hawks (Sharp-shinned Hawks (Accipiter striatus), Coopers Hawks (A. cooperi), and Northern Goshawks (A. gentilis)) captured during fall migration through southern Idaho for variations in their baseline corticosterone levels and compared those to their estimated flight energy expenditure, size and flight measurements, and migratory status. Among the three Accipiter species, I found significant differences in baseline corticosterone with the Sharp-shinned Hawk and Northern Goshawk having similar levels and Coopers Hawks having levels nearly twice as high. 1 found no significant difference in baseline corticosterone between males and females in Sharp-shinned Hawks and Northern Goshawks, but a significant difference was found between the sexes in Coopers Hawks. In all three species, I found no significant difference in baseline corticosterone between hatch year and after-hatch year hawks. Estimated flight energy expenditure showed significant differences between the Accipiters, and appears to show that Coopers Hawks are energetically at a disadvantage compared to Sharp-shinned Hawks and Northern Goshawks. I found that all the flight measurements were significantly different among the Accipiters. Male and female Sharp-shinned Hawks and Coopers Hawks showed significant differences in all measurements except aspect ratio, but there were no differences in flight measurements between the two age groups for those two Accipiters. In Northern Goshawks, I found significant differences in the all flight measurements except wing area between the single after-hatch year female and the hatch year males. My results show that size is not the only factor that determines baseline corticosterone levels in Accipiter hawks during fall migration. Flight behavior and energetics also appear to play a role, but the one factor that appeared to have the most impact was migratory status. Sharp-shinned and Coopers Hawks migrate annually, whereas, Northern Goshawks do not.