A putative hybrid sighting in California is confirmed with careful field observations of the nesting birds.
A weird bird might be a hybrid. But it could also be a color variant or an individual in bad condition. So, you have to be careful and not jump to conclusions. This is nicely illustrated by a recent case in California when people found a dead raptor on the roadside. Plumage and other measurements were intermediate between Red-tailed Hawk (Buteo jamaicensis) and Red-shouldered Hawk (Buteo lineatus), leading to the hypothesis that the bird was a hybrid. However, genetic analyses revealed that it was a Red-shouldered Hawk, probably of eastern origin. This example shows the importance of genetic testing of hybrid hypotheses. Another possibility is careful observation of the nesting site, as illustrated by another recent case.
An Intergeneric Hybrid?
In 2012, Lisa Hug reported the sighting of a putative hybrid between Red-shouldered Hawk and Common Black Hawk (Buteogallus anthracinus). This observation was questioned by some people, who suggested it might be a juvenile Common Black Hawk. The debate prompted Stan Moore to investigate the case in greater detail. He decided to observe the nest of a female Common Black Hawk in California.
These field observations revealed that the bird was frequently engaging in breeding attempts with a Red-shouldered Hawk. In 2014, for example, “Stan Moore (SM) and property owner Guy Smith observed the male Red-shouldered Hawk mounting and copulating with the perched Common Black Hawk on the morning of 8 April. SM observed a second copulation that evening. The pair also vocalized frequently to one another. SM observed the Common Black Hawk collecting nest material on 18 March, and nest-building on 8 and 11 April.”
The copulation was successful, because a nestling was observed in May. The juvenile’s plumage showed characteristics of both species. For instance, the streaked brown crown and nape are reminiscent of Red-shouldered Hawk, while the dark brown stripes on the side of the neck point to Common Black Hawk. The behavioral and morphological observations thus support the conclusion that we are dealing with an intergeneric hybrid.
A blood sample was collected in 2014, which could be used to confirm the hybridization event genetically. But I would say the evidence is already quite convincing…
Clark, W. S., Galen, S. C., Hull, J. M., Mayo, M. A., & Witt, C. C. (2017). Contrasting molecular and morphological evidence for the identification of an anomalous Buteo: a cautionary tale for hybrid diagnosis. PeerJ, 5, e2850.
Moore, S., & Coulson, J. O. (2020). Intergeneric Hybridization of a Vagrant Common Black Hawk and a Red-Shouldered Hawk. Journal of Raptor Research, 54(1), 74-80.
Thanks to Jennifer Coulson for sending me this paper, which has been added to the Accipitriformes page.