Look up Wilson’s Warbler in your favorite field guide, and you’ll find a single species, but more and more research suggests that it’s just a matter of time until you see two.

Based on coloration and size, taxonomists divide Wilson’s into three subspecies — one found along the west coast from southwestern British Columbia to southern California, a second reaching from Alaska through interior British Columbia and the Rocky Mountains to northern New Mexico, and a third that occurs from Alberta and the Northwest Territories to the east coast of Canada.

See photos of Wilson’s Warbler.

The subspecies are commonly separated into two groups, eastern and western, based on their breeding and winter ranges and migration timing and pathways. Moreover, nuclear and mitochondrial DNA analysis performed in 2011 revealed strong differences between the eastern subspecies and its two western counterparts but only subtle differences between the western pair. The genetics led the researchers to suggest that the two groups were cryptic species — that is, distinct species that have been hidden, erroneously, under one species name.

Now we learn that researchers at the National Autonomous University of Mexico have reached the same conclusion. They assembled a huge database of warbler sightings and matched the records with monthly maximum and minimum temperatures and precipitation amounts, variables known to influence avian distribution. Then they tested how much the environmental conditions that predict the distribution of warblers in the western group predicted the occurrence of birds in the eastern group, and vice versa.

The results revealed that, while warblers from both groups could occur within a considerable portion of the eastern group’s summer range, particularly in central and eastern Canada, birds from the eastern group would feel much less at home in the western area.

The eastern group is ecologically and geographically more restricted than the western group, the researchers say, a result that “should be taken into consideration for future analyses, particularly with respect to vulnerability categorization and conservation efforts.”

Read the abstract Angelina Ruiz-Sánchez, Katherine Renton, Rosario Landgrave-Ramírez, Eder F. Mora-Aguilar and Octavio Rojas-Soto (2015) Ecological Niche Variation in the Wilson’s Warbler Cardellina Pusilla Complex, Journal of Avian Biology, 46: pages 1–12. Abstract.

A version of this article appeared in our August 2015 issue. Subscribe.

Originally Published