Sunday, May 22, 2011

Lesser vs. Greater Yellowlegs

Yesterday's blog post, which showed yellowlegs in the background of the photograph, has me thinking about identifying Lesser and Greater Yellowlegs.  I took this photograph at the River Bend Nature Center in Faribault in August 2008.  When the two species are in the same pond and their sizes can be compared, you can usually tell the larger Greater (left) from the smaller Lesser Yellowlegs (right).

Size, however, is notoriously difficult to judge in solitary birds.  Note the bill shape and color.  The Lesser Yellowleg's bill is black, thin and needle-like.  The Greater Yellowleg's bill is broader at the base, bi-colored (at least in the non-breeding season), and often slightly upturned.

Most field marks, unfortunately, often overlap.  This variability inspired Sill and Sill, in their silly birding spoof, A field Guide to Little-Known and Seldom-seen Birds of North America, to add Middle Yellowlegs and the Least Yellowlegs to the North American birdlist. (This humorous book is curiously cited in the American Ornithologists' Union's opus Birds of North America, "further complications within this group are discussed by Sill et al. (1988).")

The calls of the two yellowlegs differ.  The Greater gives three or four sharp, clipped notes.  The Lesser gives two (or three) softer, shorter notes.  The differences in pitch are a more reliable field mark than the number of notes (songs are courtesy of Thayer Birding Software).

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