Tuesday, October 22, 2013

Song Sparrow

Song Sparrows are perhaps “the most polytypic passerine in North America,” this sparrow occurs with more geographical variation than almost any other bird in the world (Arcese 2002). Ornithologists argue over the exact number, but some 38 races are recognized by the American Ornithologists’ Union. These subspecies, however, are not likely to be declared as full species—their mitochondrial DNA show little variation.

Here are three of my Song Sparrow photographs from across the United States—Arlington, Virginia (top), Aberdeen, South Dakota (middle), and near the Hearst Castle on the California coast (bottom). I am no expert on Song Sparrow races, and I lack comparative material. Nevertheless, here are my best guesses as to the subspecific identity of these birds. In any case, the purpose of this post is to point out the kinds of variation seen in Song Sparrows.

The Virginia bird may be Melospiza melodia atlantica. This race is told by its gray-colored back with black streaks. Melsospiza melodia melodia, which is found across much of eastern North America, has a different-colored back. The back streaks are reddish-brown on a brown background, not on gray like the first bird. My South Dakota bird more or less fits this description and is the race expected in the state. Arcese et al. (2002), however, warns that these two subspecies are similar (although atlantica is limited to the far eastern US).

The California bird differs from the previous two. Note the black breast streaks—not brownish like on the other two birds. Notice, too, the bill, which appears to be longer and narrower than in either of the two previous photos. Finally, the bottom photo shows a bird with a brighter, reddish-brown back with black streaking—all characteristic of Melospiza melodia gouldii.

Again, these identifications are only guesses on my part. Arcese et al. (2002) summarize Song Sparrow variation by saying that, in general, darker birds breed in areas of high humidity, whereas pale races are found where humidity is lower. Northern birds tend to be large, while smaller subspecies are found in the southwest. (Note that in this post, I use the words race and subspecies interchangeably. The definition of both terms is a geographically identifiable population within a species’ range.)

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