Duelling male streamertail hummingbirds. The individual on the left is a confirmed hybrid (T. polytmus x scitulus). Credit: Rosemary Lloyd/ Macaulay Library at the Cornell Laboratory of Ornithology (ML508984951).

The curious case of two streamertail hummingbird species whose only visible distinction is the difference in the colour of their bills.

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.

Red-billed Streamertail (Trochilus polytmus). Credit: Sam Woods.

Hybrid zone

Caroline Duffie Judy and colleagues investigated the hybrid zone that separates the two hummingbird species, which is as narrow as 3.2 km.

The authors analysed 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 characterise genomic patterns.

Black-billed Streamertail (Trochilus scitulus). Credit: Caroline D Judy.

Closely related

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 hummingbirds.

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 colours, 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.

The study is published in PNAS Nexus.