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 plain in appearance, but complex in its natural history. This species is secretive to the point of disappearing during certain periods 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 early summary declared that Cassin's Sparrows “break all the ordinary rules” (1).
The species was first described by Samuel Washington Woodhouse, following collection of a single male specimen in April 1851 near San Antonio, Texas (2). 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 the Cassin's Sparrow's unusual traits is the strong annual fluctuation in abundance and range. Within the core of its distribution, breeding densities can vary widely in response to summer rainfall, whereas at the periphery of its range, singing males may 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 (3); movement of failed breeders from a particular region to attempt breeding elsewhere during the same season (4); or that males with enlarged testes observed singing on territory and birds carrying nesting materials were not actually breeding (5). Little direct evidence has accumulated to support these hypotheses. It is possible that the Cassin's Sparrow is regionally nomadic—moving widely within years to find suitable breeding conditions, even to areas outside its normal range. However, like the previous hypotheses, evidence 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 seminal monograph on the genus Aimophila (3). Breeding biology has been studied in Texas and Arizona (6, 7). Other research has investigated variation in song among individuals and populations (8, 9). The unusual pattern of molts seen in the Cassin's Sparrow has been the subject of debate (10, 11).
Much of the recent work on the species has focused on its response to human land-use and land management. This research began in the 1980s in the grasslands of southern Arizona, among other areas (12, 13, 14, 15). More recently, avian ecologists have examined the effects of grassland restoration and management practices, especially the use of herbicides, fire, and grazing to control invasive shrubs and improve grassland quality (16, 17, 18, 19).