Novel vocalizations, including song, from 2 female Cerulean Warblers (Setophaga cerulea)

Female song is more widespread than previously assumed, and is adaptive in many species; however, it may also be an aberrant, nonfunctional, behavior in others. We document novel vocalizations, including female song, from 2 paired female Cerulean Warblers (Setophaga cerulea) recorded in southern Indiana, USA, in June 2017. This is the first documentation of female song in this species. When assessed spectrographically, one female's vocalizations were similar to the typical zeet call of this species, but with appreciable differences in duration, composition, and frequency bandwidth. The other female's vocalizations, which we define as a song, had large frequency modulations as well as harmonics. The females' vocalizations resembled neither each other's nor the typical song of an adult male Cerulean Warbler. Unlike most other instances of infrequent female song in temperate-breeding passerines, these vocalizations only occurred during the incubation and nestling stages, rather than during mate acquisition. Based on their context, they appeared to function in intersexual communication with the male mate. We discuss potential explanations, both adaptive and nonfunctional, and urge researchers of this species to pay particular attention to the vocalizations of females in their study populations.