Research with American Kestrels

The American kestrel is thought to be the most numerous bird of prey in North America, and its preference for cavity nesting sites make it an ideal study bird for behavioral research. In addition, data from nationwide count surveys, such as the Breeding Bird Survey, have begun to suggest that the American kestrel’s numbers may be dropping across the North American continent. However, there has been no evidence of a decline seen in the numbers of American kestrels migrating down the west coast of California (Golden Gate Raptor Observatory, unpublished data), or genetic evidence of a decline in breeding North and Central California birds (EWommack, unpublished data).

The Mitsui Ranch is home to 43 wooden nest boxes designed to attract nesting American kestrels (Falco sparverius). Kestrels are North America’s smallest falcons and they are widespread across the continent. They can often be seen hovering over grasslands or perched on a post or wire throughout range lands and agricultural field edges. Their diet is highly varied and includes insects, small mammals, birds, and reptiles. An observer must get quite close to these birds to appreciate their strikingly beautiful plumage. Our nest box project was initiated and maintained by Elizabeth (Beth) Wommack, a doctoral candidate in the laboratory of Professor Raurie Bowie at the University of California’s Museum of Vertebrate Zoology (MVZ), and Department of Integrative Biology at Berkeley. Beth’s research centered on the question of how feather color, and patterns, of male kestrels influence their genetic fitness. In other words, do males with brighter colors and bolder or distinctive color patterns have more success at fledging more and healthier chicks and, if so, is that fitness passed through subsequent generations?

The nest boxes at Sonoma Mountain were first installed as part of a multi-site project (all in grazed or ungrazed rangelands) designed to examine how this small falcon generates and maintains its highly variable sexually dichromatic plumage coloration. Along with questions on behavior, the establishment of a long-term nest box colony at Sonoma Mountain will provide an important window into the yearly breeding patterns of the species. The genetic, morphological, and banding data collected at Sonoma Mountain will provide a crucial set of information for attempting to understand the relationship between the American kestrel and rangelands in Northern California.