Cassin’s Sparrows, plain, dull-colored and often elusive birds in their grassland habitats at higher elevations on the Great Plains and desert southwest, are most easily detected when the males engage in song flights during mating season. This behavior is used by a variety of species associated with grassland or low shrub breeding habitat to compensate for the lack of suitable song perches (Dunning et al. 1999).
The behavior of Cassin’s Sparrows is enigmatic in several respects, with the large variations in breeding populations in individual areas being the most puzzling. While summer rainfall seems to be connected with breeding activity and population size, the ,mechanisms involved remain unclear (Dunning et al. 1999).
DISTRIBUTION. During the 1987-1992 field work seasons of the TBBA project, researchers found confirmed breeding evidence for Cassin’s Sparrows in 104 blocks, probable evidence in 488 blocks and possible evidence in 133. Most blocks are in the western 2/3 of Texas in the High Plains, western Rolling Plains, Edwards Plateau, South Texas Brush Country, Trans-Pecos and Coastal Sand Plain eco-regions (see the region map in Lockwood and Freeman ). In Oklahoma atlasers found breeding evidence in the western 1/3 of the state (Carter and Duggan 2004).
Cassin’s Sparrows also breed in southeast Arizona, New Mexico, eastern Colorado, western Kansas and Nebraska, southeast Wyoming and the highlands and Gulf Coastal plain of northern Mexico. Breeders in Colorado, Kansas, Oklahoma, northern New Mexico and the Texas Panhandle move southward within the breeding area and to the western coast of the Gulf of California in winter (Howell and Webb 1995, Melcher 1998, Dunning et al. 1999, Corman 2005, (Sauer et al. 2008).
SEASONAL OCCURRENCE Most Cassin’s Sparrows are only present in Texas during the breeding season. In central Texas, some males remain throughout winter, but a “conspicuous influx” of migrants arrives to claim territories in early March (Schnase 1984).
Cassin’s Sparrows breed in Texas from early March through August, based on egg collection dares from March 1 to August 1. Most breeders are gone by the end of October, although these sparrows are uncommon to rare residents in winter in the Trans-Pecos, western Edwards Plateau and South Texas Brush Country (Oberholser 1974, Lockwood and Freeman 2004).
BREDING HABITAT. Cassin’s Sparrows breed in Texas from near sea level to 1500 m (5000 ft) primarily in short-grass prairies with scattered shrubby mesquite, cacti, yucca or oak (Oberholser 1974). In Arizona 79% of breeding evidence was found in semi-arid grassland, often with scattered trees and shrubs (Corman 2005). On the eastern plains of Colorado 55% of breeding evidence came from short-grass prairies with another 38% from other grassland habitats (Melcher 1998). In Mexico these sparrows breed in grasslands with scattered bushes or open brushlands (Howell and Webb 1995).
Cassin’s Sparrow nests, constructed by the female, are usually placed on the ground at the base of a small bush or grass clump, or are placed low in a small shrub. The nest is built of dry grass blades, forb stems and vegetable fibers and lined with fine grasses or hair. The outside diameter is about 10 cm (4 in), inside diameter 6.3 cm (2.5 in; Harrison 1979, Schnase1984, Dunning et al. 1999).
In Texas and Oklahoma females usually lay 4-5 white, unmarked eggs. The nest and eggs are similar in appearance to those of Botteri’s Sparrow (Aimophila botterii). Incubation starts after the laying of the next to last egg and lasts 11 or more days. The nestlings fledge about 9 days after hatching, stay in dense cover with their mother for about a week and become completely independent within a month after hatching (Harrison 1979, Schnase1984, Schnase et al.1991, Dunning et al. 1999).
STATUS. Lockwood and Freeman (2004) describe Cassin’s Sparrows as common to abundant summer residents of the Panhandle and South Plains, and scarcer in the remainder of the western 2/3 of Texas. North American Breeding Bird Survey (BBS) data for 1994-2003 indicate a relative abundance of 30-100 sparrows per 40 km (25 mi) route for the northwest Panhandle and western Edwards Plateau. Areas with this same relative abundance were found in eastern New Mexico and Colorado (Sauer et al. 2008). The general similarity of the distribution of breeding and summer symbols on the map in Oberholser (1974) to the TBBA map below suggests no substantial change in the range of this species has occurred in Texas in recent years. BBS data for 1980-2007 from 126 routed in Texas \show a statistically significant annual population change of -1.5% (Sauer et al. 2008), probably not too worrisome because of the high relative abundance. Text by Robert C. Tweit (2009)
Carter, W. A. and M. D. Duggan. 2004. Cassin’s Sparrow (Aimophila cassinii) . In Oklahoma breeding bird atlas, pp. 394-395 (D. L. Reinking, ed.). University of Oklahoma Press, Norman.
Corman, T. E. 2005. Cassin’s Sparrow (Aimophila cassinii). In Arizona breeding bird atlas. pp. 504-505 (T. E. Corman and C. Wise-Gervais, eds.). University of New Mexico Press, Albuquerque.
Dunning, Jr., J. B., R. K. Bowers, Jr., S. J. Suter and C. E. Bock. 1999. Cassin’s Sparrow (Aimophila cassinii), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/471
Harrison, H. H. 1979. A field guide to western birds’ nests. Houghton Mifflin, Boston, MA.
Howell, S. N. G. and S. Webb. 1995. A guide to the birds of Mexico and northern Central America. Oxford University Press, New York.
Lockwood, M. W. and B. Freeman. 2004. The TOS handbook of Texas birds. Texas A&M University Press, College Station.
Melcher, C. 1998. Cassin’s Sparrow (Aimophila cassinii). In Colorado breeding bird atlas, pp. 450-451 (H. E Kingery, ed.). Colorado Bird Atlas Partnership, Denver.
Oberholser, H. C. 1974. The bird life of Texas. University of Texas Press, Austin.
Sauer, J. R., J. E. Hines, and J. Fallon. 2007. The North American breeding bird survey, results and analysis 1966-2006. Version 7.23.2007. USGS Patuxent Wildlife Research Center, Laurel MD < http://www.mbr-pwrc.usgs.gov/bbs>
Schnase, J. L. 1984. The breeding biology of Cassin’s Sparrow (Aimophila cassinii) in Tom Green County, Texas. M.S. thesis, Angelo State Univ., San Angelo, TX. .
Schnase, J. L., W. E. Grant, T. C. Maxwell and J. J. Leggett. 1991. Time and energy budgets of Cassin’s Sparrow (Aimophila cassinii) during the breeding season: evaluation through modeling. Ecol. Modeling 55: 285–319. .