Wednesday, December 14, 2011

Western vs. Clark's Grebes

Western Grebes (the top photograph) are locally common in the summer in western Minnesota; Clark's Grebes are also found in the western part of the state, but are rare. (The ratio of the two species in the eastern Dakotas is about 100 Westerns to one Clark's.) I usually see several Western Grebes in Rice County, but, to date, never a Clark's. (These photographs were taken at Sand Lake National Wildlife Refuge in northeastern South Dakota.)

At least four field marks can be used to separate these two grebes. 1) The Clark's Grebe's bill is brighter yellow than the Western's. 2) The Clark's Grebe's eye is surrounded by white, the Western's is surrounded by black. 3) The Clark's Grebes flanks and back tend to be much paler than those of the Western. 4) Clark's Grebes' calls usually have one syllable, while Westerns have two notes. Be forewarned, however, that these grebes are variable in color and plumage, and some can be very difficult to identify. Furthermore, the two species occasionally interbreed, creating intermediate hybrids.

Until recently these two species were considered to be color morphs of a single species. Both grebes perform identical, complex courtship displays. So why are they now thought to be distinct? Western and Clark's Grebes prefer not to breed with each other. In the Dakotas, male Westerns apparently interbreed with Clark's only out of desperation late in the season after they are unable to find an appropriate female. Although DNA sequences are similar in the two species, the differences are of equal magnitude as other closely related species. The difference in the number call call notes is apparently critical in keeping Western and Clark's Grebes reproductively isolated (Storer and Nuechterlein 1992).

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