First confirmed hybrid pairing between a Cerulean Warbler (Setophaga cerulea) and a Black-throated Blue Warbler (Setophaga caerulescens)

Abstract

Hybridization events have long interested biologists because of their implications for species concepts and taxonomy. Documenting hybridization events is important because hybridization rates are commonly used as support for accepted taxonomic divisions. In July 2017, we observed a nest where a male Cerulean Warbler (Setophaga cerulea) and a female Blackthroated Blue Warbler (Setophaga caerulescens) were feeding nestlings. The Cerulean Warbler and Blackthroated Blue Warbler communicated with each other using song (male) and calls (female), similar to a nonhybrid pair of either parent species, and the pair was not observed to engage in aggressive behaviors toward each other. Interestingly, the Black-throated Blue Warbler is not known to breed in Indiana, although it is a regular migrant in the spring and fall. We describe the behaviors of the hybrid pair at the nest, provide photographs and audio recordings for documentation, and hypothesize that this instance of hybrid pairing may have occurred due to mate scarcity.