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Male Townsend's warbler
National Parks Service

He might seem handsome and sweet, but don’t be mistaken—the Townsend’s warbler is a bully who will steal another guy's girlfriend. That’s what dogged detective work by University of Washington doctoral student Meade Krosby has revealed, helping to solve the cold case of the disappearing hermit warbler.

This drama began 400,000 years ago when glaciers spliced the range of a single warbler species. Over time, the separated populations evolved to become two unique species: Townsend’s warblers, marooned inland in Oregon, Washington, Idaho, and Montana; and hermit warblers, basking on the Pacific coast from northern California to Alaska.

The Townsend’s warbler wound up evolving with pumped up testosterone levels and advanced fighting capabilities. When the glaciers melted, the Townsend's pushed north to British Columbia and Alaska, and eventually the Pacific Coast, where it gained the upper hand over the hermit and began taking over its northern range.

DNA evidence uncovered by Krosby in the lab showed that the key to their success was the macho Townsend’s males' ability to woo and mate with hermit females. In time their progeny took on all of the characteristics of the Townsend’s warbler, dominating the coast from northern Washington to Alaska, along with the species' original inland range. The hermit warblers retreated south, and remain today in a smaller range within California, Oregon, and Washington. Lingering hybrid zones still exist in Oregon and Washington. 

Krosby, who published her research in the most recent edition of Proceedings of the Royal Society B, described the discovery as a “genetic smoking gun” that shows what the Townsend’s warbler did to its “sister species.” Case closed.