Differential migration and phenology of adult Red-tailed Hawks in California

The following story highlights a raptor research project that tells us something new about the incredible birds of prey we monitor at the Golden Gate Raptor Observatory (GGRO). Thank you to our GGRO volunteers for preparing this summary. Read more raptor science stories here >>

Authors: Katharine M. Tomalty, Angus C. Hull, Allen M. Fish, Christopher W. Briggs, and Elisha Hull

Year: 2016

Journal: Journal of Raptor Research

Red-tailed Hawks are one of the most ubiquitous species of raptor in the San Francisco Bay Area. Despite their abundance, one impediment to the study of Red-tails has been the difficulty of determining the sex of individual hawks counted and banded at GGRO. Like most raptors that pass through the Marin Headlands, Red-tails of different sexes show no difference in feather coloration to help researchers identify them. As a general rule, female raptors are larger than males, but this size difference is not as pronounced in Red-tailed Hawks as some other common species. Furthermore, Red-tails in different parts of North America have separate breeding populations and size can differ across regions.

Katherine Tomalty of UC Davis and her colleagues at GGRO used DNA collected from feather samples of adult Red-tails banded at GGRO to determine the sex of individual birds. The authors then compared the standardized weight, talon, leg, and beak measurements collected by GGRO volunteer banders from those same birds to develop a range of values for each measurement which could be used to accurately determine the sex of adult Red-tails captured at GGRO. DNA analysis of feather samples carries a significant financial cost—the ability to determine the sex of a Red-tail from standard measurements already taken during the banding process allowed the researchers to efficiently determine the sex of any adult Red-tail which had previously been banded at GGRO.

Previous research published by GGRO and UC Davis showed that juvenile Red-tailed Hawks migrating through the Marin Headlands have two distinct peaks. The first peak, in mid-September, is comprised mostly of local hawks dispersing from their parent’s nesting territories, while the second peak, occurring in mid-November, is comprised mostly of hawks from the Intermountain West. Tomalty and her colleagues completed the same analysis for adult birds and found that, unlike juveniles, the number of adult Red-tails banded at GGRO increases gradually throughout the fall. This may reflect the fact that much of coastal California provides good conditions for Red-tails year-round, so adult Red-tails that breed locally are not likely to disperse from their nesting sites when their offspring do early in the fall. However, Red-tails migrating to the Pacific coast from the Intermountain West later in the fall are more likely to include both juvenile and adult birds.

When the researchers looked for a difference in the timing between the sexes of adult Red-tails, they found that females tended to migrate earlier than males. This mirrors the results of similar analyses of differences in migration timing performed on other species of raptors at GGRO. This important research helps us to better understand the complex ecology of one of our most common raptor species, and opens the door for future research that could explore the causes of these differences in migration timing.

Read the full paper here >>