Worthen’s Sparrow: What is it?

Worthen’s Sparrow (Spizella wortheni) has been a creature of mystery ever since its discovery.  It was first found in the southwestern United States, near Silver City, New Mexico, in June 1884, but it has never been found north of the border since then.  Its total known range today is a limited area on the northern central plateau of Mexico.  I have searched for the species several times but have seen it only once, in early spring 2005, when Kimberly and I went with Adrian Ganem Sada and other Mexican ornithologists to look at a known colony southwest of Monterrey.  Worthen’s Sparrow has disappeared from most of its former known range and is now considered seriously endangered — in fact, its total population may now be fewer than 200 individuals.  So it’s of great interest to conservationists as well as birders.

At first glance or in illustrations, Worthen’s Sparrow looks a lot like a Field Sparrow — and in fact, the photo at the top of this post is a photo of a Field Sparrow, because I don’t have any shots of Worthen’s!  But in the field it doesn’t give the same impression, and its song is very different.  Ornithologists have long assumed that it was a close relative (or even a subspecies) of Field Sparrow, just because of its superficial similarity.  But now there’s molecular evidence that suggests otherwise.  Ricardo Canales del Castillo and others have just published a paper in Molecular Phylogenetics and Evolution reporting on a genetic study of this and the other Spizella sparrows.  Despite its similarity to Field Sparrow, Worthen’s is apparently more closely related to the very dull-colored Brewer’s Sparrow, a bird that nests on sagebrush plains of the western U.S. and Canada.  A link to the abstract of the article is here.