A Cassin's Sparrow meets its end a long way from home
Every day during fall migration, the Chicago Bird Collision Monitors deliver to the Field Museum a bag of birds that died flying into windows in the loop as they tried to make their way south to their wintering grounds farther south. These salvaged birds provide a critical component of the museum's bird collection, specimens that can be used by researchers for generations to come to learn about many aspects of our area's birdlife. It's really the common birds, species like White-throated Sparrow and Yellow-bellied Sapsucker and Ovenbird, that provide the backbone of this collection, but every once-in-a-while something utterly unexpected arrives in the prep lab.
September 8 was one of those days. When Dave Willard started his grisly morning ritual of cataloging the day's dead birds, he opened one of the plastic bags and pulled out a small, non-descript brown bird. With his practiced eye, he immediately realized that the bird was not one of the usual suspects. In fact it was a sparrow of the genus Aimophila, which contains four difficult-to-identify species in North America, none of which are supposed to occur anywhere near Chicago.
Dave took the specimen into the museum's bird collection--one of the world's largest and most complete--to compare it with study skins of the various Aimophila sparrows; his suspicion was confirmed--it was a Cassin's Sparrow (Aimophila cassinii), a bird typical of dry grasslands of the southwestern United States and northern Mexico. When Ben Marks prepared the specimen, he found that, like many vagrant birds--those that end up far outside their normal range--it was an immature, born during the preceding summer. Cassin's Sparrows have a history of vagrancy, showing up far-flung places with some regularity. This was the first specimen of the species from Illinois, and just the third time the species has ever been found in the state, according the Field Museum's Doug Stotz. The first was in Olive Park, along Chicago's lakefront, in May-June 1983 and the second was in Winthrop Harbor, near the Wisconsin border, in May 2011.
