A wood-warbler produced through both interspecific and intergeneric hybridization

Abstract

Hybridization between divergent taxa can provide insight into the breakdown of characters used in mate choice, as well as reproductive compatibility across deep evolutionary timescales. Hybridization can also occur more frequently in declining populations, as there is a smaller pool of conspecific mates from which to choose. Here, we report an unusual combination of factors that has resulted in a rare, three-species hybridization event among two genera of warblers, one of which is experiencing significant population declines. We use bioacoustic, morphometric and genetic data, to demonstrate that an early generation female hybrid between a golden-winged warbler (Vermivora chrysoptera) and a blue-winged warbler (V. cyanoptera) went on to mate and successfully reproduce with a chestnut-sided warbler (Setophaga pensylvanica). We studied the product of this event-a putative chrysoptera × cyanoptera × pensylvanica hybrid-and show that this male offspring sang songs like S. pensylvanica, but had morphometric traits similar to Vermivora warblers. The hybrid's maternal parent had V. chrysoptera mitochondrial DNA and, with six plumage-associated loci, we predicted the maternal parent's phenotype to show that it was likely an early generation Vermivora hybrid. That this hybridization event occurred within a population of Vermivora warblers in significant decline suggests that females may be making the best of a bad situation, and that wood-warblers in general have remained genetically compatible long after they evolved major phenotypic differences.

Keywords: birds; hybridization; reproductive isolation.

Figure 1.

(a) The predicted formation of this new hybrid. (b) A female Brewster's in 2017 at the site where the 2018 hybrid (c) was captured. Vermivora illustrations by Liz Clayton Fuller, chestnut-sided warbler from [20]. Photos in (b,c) by L.B.

Figure 2.

(a) Culmen and wing length for Vermivora warblers (yellow, golden-winged; blue, blue-winged; grey, Brewster's) and chestnut-sided warblers (red). The hybrid is shown with an asterisk. (b) The genetic index across six loci that distinguish Vermivora warblers. From the hybrid, we estimated the genetic index of its maternal parent, falling within the range of hybrids between golden-winged and blue-winged warblers (i.e. ‘Brewster's warblers’).