Date of this Version
The Nebraska Bird Review, Vol. 89 No. 1, pp. 41-45.
The Townsend’s Warbler (Setophaga townsendi), Hermit Warbler (Setophaga occidentalis), Golden-cheeked Warbler (Setophaga chrysoparia), and Black-throated Green Warbler (Setophaga virens) are similar in appearance and vocalizations. Evaluation of mtDNA suggests that these four species diverged about 1 million years ago during the mid-Pleistocene (Lovette et al. 2010), at which time glaciation separated ancestral forms thus allowing for speciation (Morse and Poole 2020). Under the environmental conditions prevalent today, however, the breeding distribution of Townsend’s Warbler now overlaps to a limited extent with those of the Hermit and Black-throated Green Warblers, and hybridization occurs frequently in these contact zones. The contact zone between Townsend’s and Hermit Warblers in Oregon and Washington has been studied extensively (Wright et al. 2020) and several hybrid individuals have even strayed to the southern Great Plains. Recently for example, Townsend’s x Hermit Warblers were photographed in Hamilton Co., Kansas on 17 May 2014; Midland Co., Texas on 17 Sep 2017; Pueblo Co., Colorado from 10-11 Sep 2020; and in Roosevelt Co., New Mexico between 26 Sep and 4 Oct 2020 (eBird 2021).