Sex and age group specific changes in body condition of red-tailed hawks in Central Nebraska

The central plains and the Platte River Valley of Nebraska are recognized as important winter habitat for many raptor species, but few studies have evaluated the ecology or physiology of birds of prey wintering in the region. The purpose of our study was to collect morphological data from Red-tailed Hawks (Buteo jamaicensis) occupying the study area during non-breeding months to better understand the sex ratios and potential changes in body condition of raptors in central Nebraska. Female Red-tailed Hawks were trapped significantly more often than males (κ² = 11.560, df = 1, P < 0.01), and data suggest that the population consists of a larger number of adult females followed by juvenile females and adult males, respectively. No juvenile males were captured during the study. Stratifying the data by sex and age group indicate that adult female Red-tailed Hawks significantly increase body mass (F = 13.049, df = 1, 13, P = 0.004) over time, and normalization of data by animal size to form a body condition index suggests the increase in mass resulted from the accumulation of energy reserves. Juvenile female and adult male mass did not significantly change with time during the study. We propose that these data support adult females occupying the study area at higher frequencies than males or other age groups, and that the region may be important to adult females for accumulating energy reserves to meet the challenges of the upcoming reproductive cycle.