Cassin's Sparrow is a somewhat elusive resident of arid shrub grasslands of the southern High Plains, the southwestern United States, and northern Mexico. It is dull and plain in appearance, but complex in its natural history. This species is secretive to the point of disappearing during certain parts of the year but is a distinctive and easily identified bird when males are singing and performing their song-flights. Many populations increase breeding activity dramatically when summer rains begin, but this response varies from location to location. Migratory status, seasonal movements, and even molt patterns are unusual for North American songbirds. One prominent summary declares that Cassin's Sparrows “break all the ordinary rules” ( Phillips et al. 1964a ).
The species was first described by S. W. Woodhouse, following collection of a single male specimen near San Antonio, Texas, on 25 April 1851 ( Woodhouse 1852 ). He named the new species in honor of his friend John Cassin, then curator of birds at the Academy of Natural Sciences in Philadelphia. Most people identify their first Cassin's Sparrow during the breeding season, when territorial males are “skylarking,” a distinctive flight display given while singing.
Perhaps the most dramatic of Cassin's Sparrow's unusual traits are the strong annual fluctuations in numbers and range. Within the heart of its distribution, breeding densities can vary widely in response to summer rainfall, while at the periphery of its range singing males can suddenly appear (often in large numbers) in some years at locations where no birds usually breed. A series of unusual hypotheses have been proposed to explain these fluctuations: an east-to-west migration pattern unique among North American sparrows ( Wolf 1977 ); movement of failed breeders from a particular region to attempt breeding elsewhere during the same season ( Hubbard 1977c ); or that males with enlarged testes observed singing on territory and birds carrying nesting materials were not actually breeding ( Phillips 1944b ). Little direct evidence has accumulated to support these hypotheses. It is possible that Cassin's Sparrows are regionally nomadic—moving widely within years to find suitable breeding conditions, even to areas outside their normal range. However, like the previous hypotheses, proof for nomadism remains circumstantial.
Although this species has not been thoroughly researched in many regards, several published studies provide important data. Information on body measurements, natural history, and systematics was summarized in Wolf's ( Wolf 1977 ) seminal monograph on the genus Aimophila . Breeding biology has been studied in Texas and Arizona ( Maurer et al. 1989 , Schnase et al. 1991 ), while response of the species to human land-use has been the subject of Arizona research (Bock and Bock Bock and Bock 1988 , Bock and Bock 1992 , Bock and Webb 1984 , Lloyd et al. 1998 ). Variation among individuals and populations in song has been studied by Borror ( Borror 1971 ) and Schnase and Maxwell ( Schnase and Maxwell 1989 ). The unusual pattern of molts has been the subject of debate ( Willoughby 1986 , Thompson and Leu 1994 ).