Birders and scientists have long recognized two subspecies of Willet — and for many reasons.

The breeding populations of each are widely separated. The eastern subspecies nests in coastal marshes primarily along the Atlantic and Gulf coasts, while the western subspecies nests in wet prairies and grasslands in the western interior of the continent. The two subspecies sometimes co-mingle during migration, and the birds may also share wintering grounds in some areas, such as along the coasts of Central and South America.

Geography aside, Eastern and Western Willets also differ physically. Eastern Willets are about 10 percent smaller than Western Willets. Their legs, necks, and wings are shorter, giving a generally more slender appearance, and their bills are a bit shorter and narrower than their western-breeding counterparts. Moreover, in winter plumage, Eastern Willets are darker and more heavily patterned than the paler western subspecies.

Finally, although differences are slight and require a good ear, the “songs” of the two populations are different. Eastern Willets make shorter, higher-frequency vocalizations more suited to be heard over the surf of their coastal breeding habitats.

The differences have led some taxonomists to suggest that the Willet should be split into two separate species. Now, thanks to a team led by Jessica Oswald, they have physical and molecular data to bolster that argument. Oswald is a postdoctoral researcher at the Florida Museum of Natural History, in Gainesville. Between August 2014 and May 2016, she was the vertebrate paleontology postdoctoral researcher at the Louisiana State University Museum of Natural Science.

The researchers analyzed the structural differences between the subspecies and performed genetic tests. In a recent paper in The Auk: Ornithological Advances (abstract), they report that the anatomical attributes noted above differentiate the subspecies 94 percent of the time, and that genetic tests detected no hybridization or genetic introgression between the two subspecies. Indeed, the two forms appear to have been on their own evolutionary paths for at least 300,000 years.

Taken together, the geographic, morphological, ecological, and genetic data strongly support the argument that Eastern Willet and Western Willet are two distinct species. — Julie Craves

A version of this article appeared in the January-February 2017 issue of BirdWatching.

Read the abstract Jessica A. Oswald, Michael G. Harvey, Rosalind C. Remsen, DePaul U. Foxworth, Steven W. Cardiff, Donna L. Dittmann, Libby C. Megna, Matthew D. Carling, and Robb T. Brumfield (2016) Willet Be One Species or Two? A Genomic View of the Evolutionary History of Tringa semipalmata. The Auk: Ornithological Advances: October 2016, Vol. 133, No. 4, pp. 593-614. Abstract.

View reader photos of Willet.

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Originally Published