This study was designed to assess northern goshawk (Accipiter gentilis) reproduction and adult health in relation to central Idaho habitat quality. Productivity was measured at 34 nests during 1998 and 1999. Mean productivity was 2.1 fledglings per successful nest in 1998 and 2.0 fledglings per successful nest in 1999. In both years 1.5 young fledged per occupied nests. The nest failure rate in 1998 was 17% and 21% in 1999. Nestling growth, measured in days to reach 90% of fledging mass, ranged from 16 to 35 days for 24 nestlings. The fastest mean growth at a nest was 16 days to reach 90% while the slowest mean growth was 33 days to 90% of fledging mass.
I used mean levels of seven metabolic substances to describe metabolic health in the adult breeding goshawks. Levels of protein, cholesterol, calcium, uric acid, aspartate aminotransferase, lactate dehydrogenase, and creatine kinase were variable among the 29 adults sampled but comparable to data reported for other raptors. There were no significant differences between males and females in these substances.
Corticosterone, the stress response hormone, was measured in two blood samples to index the stress response to capture and handling. Both increasing and decreasing levels of corticosterone were observed over a 30-minute holding period. These variable results may be due to individual variation, complications involved in trapping breeding goshawks, or stress modulation during the breeding season.
An alternative measure of stress was determined from differential white blood cell counts. The ratio of heterophils to lymphocytes has been used as a measure of chronic stress in birds and was calculated for 29 goshawks in this study. Heterophil/lymphocyte ratios were not correlated with corticosterone in the goshawk samples. Total white blood cell estimates provided a representation of immune system health and were within ranges reported for captive goshawks. Blood parasites were identified along with white blood cells and were common among the sampled goshawks.
A body condition ratio was calculated using mass, wing length, tail length, and culmen. Body condition results indicated that only birds in average to good condition were breeding. I also assessed body condition on the basis of muscle mass and fault bars and found little significant variability. Of all the physiological measures only body condition was related to nestling growth. No adult health variables were related to productivity.
Finally, all reproductive and adult health data were used to validate the results of a goshawk habitat potential model. The model was developed for the Intermountain West and scored habitat based on vegetation and landform features at the nest area and post-fledging area scales. The model also identified and scored suitable 1 ha nests sites. All occupied nest sites from this study were within the range of suitable sites identified by the model.
Productivity results from 25 nests were not associated with habitat quality at either scale. Nestling growth rates at 12 nests were lower as habitat quality increased at both scales inverse to the expected relationship. Adult health measures showed variable associations at both scales. Male protein levels increased with increasing habitat quality, as expected, and female white blood cell estimates and heterophil/lymphocyte ratios decreased with increasing post-fledging area quality as would be expected. A study including home range quality assessment, more replicates, and additional years of data collection would improve the reliability of the model validation.
Hanauska-Brown, L.A., A.M. Dufty Jr., and G.J. Roloff. 2003. Blood chemistry, cytology, and body condition in adult Northern Goshawks (Accipiter gentilis). Journal of Raptor Research 37:299-306.
Hanauska-Brown, L.A., M.J. Bechard, and G.J. Roloff. 2003. Northern Goshawk breeding ecology and nestling growth in mixed coniferous forests of west-central Idaho. Northwest Science 77:331-339.