A recent study has documented the existence of a previously undiscovered hybrid zone between two hummingbird species in northern California and southern Oregon.
Species are usually though of as being reproductively isolated – that is, not mating with other species in the wild. Occasionally, however, closely related species do interbreed in an area where their ranges overlap, in what is known as a hybrid zone. In this instance, two hummingbirds – Allen's and Rufous – are blurring species boundaries.
To map the extent of the hummingbird hybrid zone in northern California and southern Oregon, San Diego State University's Brian Myers and his colleagues collected data on the physical traits and courtship behavior of more than 300 hummingbirds in the region. Most of the breeding males across the hybrid zone had a mix of characteristics of the two species, shifting gradually from more Rufous-like birds in the north to more Allen's-like birds in the south.
A hybrid Allen's × Rufous Hummingbird in California (Brian Myers)
The males of different hummingbird species have distinct displays, performing aerial acrobatics during which their tail feathers produce various sounds. The researchers captured hummingbirds using traps at feeders, temporarily keeping females in mesh cages, where they caught the attention of territorial males. "Sometimes the birds outsmart me," said Myers. "They'll only visit a feeder when the trap isn't on it, or they won't perform their courtship displays to the female hummingbird I'm carrying around, and this can make things very slow sometimes."
The area where Allen's and Rufous Hummingbirds hybridise stretches more than 300 km along the Pacific coast and 90 km inland, and it could have implications for the species' futures. Myers explained: "When a hybrid zone is so large, and when one of the hybridizing species has as small a range as Allen's Hummingbird, it raises the possibility of their range shrinking even further as they're swamped by hybrids that carry Rufous Hummingbird traits and pass these genes into Allen's Hummingbird populations.
"As biodiversity continues to drop, it's more important than ever to understand how new species form and what maintains species barriers once they're made – is there a certain habitat or other resources that require protection? Is it more related to sexual selection? Hybrid zones are an ideal tool with which to study this."
Myers, B M, Rankin, D T, Burns, K J & Clark, C J. 2019. Behavioral and morphological evidence of an Allen's × Rufous hummingbird (Selasphorus sasin × S rufus) hybrid zone in southern Oregon and northern California. The Auk, Volume 136, Issue 4, 1 October 2019, ukz049, https://doi.org/10.1093/auk/ukz049