Tuesday, July 16, 2013

Killdeer and Horned Lark

A wide range does not necessarily mean a bird will have multiple races. Killdeer (above) are found from Alaxka to Newfoundland, south to Mexico and from the Bahamas through much of the West Indies. Other breeding populations are found in Costa Rica, Aruba, and coastal Peru and Chile. Across North America, little variation is found—northern birds are a bit larger than southern ones; West Indian breeders are a bit grayer than others. Only one North American race is recognized by ornithologists—Charadrius vociferus vociferus. The South American and West Indian birds are each given subspecific status. But we are warned by Jackson and Jackson (2000) that these non-North American races are of dubious validity; they are only slightly different and require “modern appraisal.”

Compare the Killdeer’s lack of variation with that of another wide-spread North American bird, the Horned Lark. The Horned Lark breeds across most of North America. Body size and coloration vary by breeding location. They superciliary stripe varies from white to yellow. The back color is “strongly correlated with the color of local soil” (Beason 1995). (See also my previous Horned Lark post.) Note the differences between the Horned Larks in the lower photos. The left one was taken this spring near Northfield, Minnesota. The one on the right was in California. Ornithologists divide the species into 21 races, most intergrading into adjacent subspecies. At least two races breed in Minnesota, Eremophila alpestris praticola and Eremophila alprestris leucolaema, the first in most of the state, the second in the far west. Other races undoubtedly visit Minnesota in the winter.

Why the difference in variation in the two birds? A likely hypothesis is that Horned Larks faithfully return to their natal regions whereas Killdeers may be less predictable. As such, the Killdeer may mix genes among themselves at a much greater frequency than do the larks.

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