Intergeneric Hybridization of a Vagrant Common Black Hawk and a Red-Shouldered Hawk

Abstract

Natural intergeneric hybridization occurs rarely in raptors. In 2012, L. Hug observed a juvenile hawk interacting with a banded adult Common Black Hawk (Buteogallus anthracinus) in Sonoma County, California, USA, and reported that the juvenile was a Common Black Hawk × Red-shouldered Hawk (Buteo lineatus) hybrid (Hug 2016; Western Birds 47:325–326). A disparity of opinions concerning the parentage of the juvenile prompted us to document the nesting efforts of this Common Black Hawk in 2013 and 2014. The 2013 nesting attempt failed during incubation before the identity of the mate could be confirmed. In 2014 the adult female Common Black Hawk bred with an adult male Red-shouldered Hawk (B. l. elegans), and this pair fledged one hybrid offspring. Evidence of this hybridization included observations of the pair engaged in courtship flights (n = 3), copulations (n = 2), and simultaneous nest attendance (n = 2), and the offspring's intermediate physical characters and vocalizations. Although the hybrid offspring's juvenile plumage more closely resembled a juvenile Common Black Hawk overall, the hybrid also exhibited some intermediate plumage, morphological, and vocal characteristics. This hybridization event appears to be an example of the desperation hypothesis, in which a rare vagrant, unable to secure a conspecific mate, settled on a species of raptor abundant in western California. Dietary overlap of the two parental species (e.g., crayfish, amphibians) and strong associations with aquatic habitats may also have facilitated this pairing and hybridization event.