Thursday, August 18, 2011

Northern Rough-winged Swallow

Last week, Erika and I canoed the Cannon River from Northfield to Cannon Falls. Along the way we saw a few Northern Rough-winged Swallows as they perched on exposed timber over the river. This swallow breeds across North America, from southern Canada through most of the United States. Our birds migrate south in the winter to join resident populations from Mexico through much of Central America.

Why is this bird called a Northern Rough-winged Swallow? The outer-most primary of this swallow has a rough edge. In adult birds, the outer primary edge lacks barbules. Males have curved hooklets and females have thickened spines that give the feather a rough edge. De Jong (1966) writes, "The possible adaptive significance of this feature remains a mystery."

Why is this bird called a Northern Rough-winged Swallow? Rough-winged Swallows were thought to range as groups of interbreeding races from North America through South America. In 1981, researchers in Costa Rica discovered a break in this species's range. Northern birds breed in the mountains, whereas southern birds are restricted to lowlands. Formerly considered to be races and although very similar in appearance, northern and southern birds can be told apart.  Since they do not interbreed, we have the Northern Rough-winged Swallow. The Southern Rough-winged Swallow is found from Costa Rica on south.

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