Hummingbird Bills: Tale of Two

There are two species of streamertail hummingbirds on the island of Jamaica, West Indies—one with red-billed males (Trochilus polytmus) and the other with black-billed males (T. scitulus). This is a puzzling situation, as many evolutionary biologists have argued that avian speciation is unlikely to occur on small oceanic islands. Caroline Duffie Judy and colleagues investigated the hybrid zone that separates the two species, which is as narrow as 3.2 km. The authors analyzed 186 Trochilus specimens from 12 sample locations across the island, using 6,451 single nucleotide polymorphisms (SNPs) and a piece of the mitochondrial control region to characterize genomic patterns. The two species are closely related, suggesting either a recent speciation event or extensive gene flow following secondary contact. Although a river separates the species, the waterway is unlikely to serve as a significant barrier to the birds. Instead, the authors propose that sexual selection is likely to play a key role in both driving and maintaining species differentiation. According to the authors, hybrids, which have mottled bill colors, may be less attractive to females of either species because their bills resemble those of juveniles—a mechanism that may contribute to the extremely stable hybrid zone.