Breeding Habitat:Grassland
Nest Type:Open-cup nesting passerine
Migration Status:Short distance migrant
Nest Location:Ground-low nesting
During the breeding season, Cassin's Sparrows inhabit short-grass prairies mixed with scattered shrubs. They avoid pure grassland and pure shrubland habitats. Their populations are known to experience considerable annual fluctuations in abundance, primarily in response to changes in precipitation levels. In Colorado, Cassin's Sparrows are least numerous during wet years when the grassy vegetation becomes too tall (Andrews and Righter 1992). In the southwestern deserts, however, they are generally most numerous during wetter years but become scarce during droughts.
Along BBS routes, Cassin's Sparrows are most numerous from southeastern Colorado south through eastern New Mexico and adjacent portions of Texas (Relative Abundance Map). They become increasingly scarce towards the periphery of their range. This map reflects their distribution and relative abundance during June, when most surveys are conducted. A different distribution pattern would be evident later in summer, when nonbreeding birds move into the Sonoran Desert of Arizona and adjacent areas with the advent of the late summer rains (Phillips 1944).
As a result of the considerable annual fluctuations in abundance, the historic changes in Cassin's Sparrow populations are poorly understood in most of their range. This species is thought to have increased in portions of Texas since the 1930s, a result of brush removal activities (Oberholser 1974). Similar trends have not been reported elsewhere.
The BBS trend estimates for this species should be viewed with caution, since years of unusual abundance or scarcity could markedly influence these estimates. Over the entire survey period, significant declines in the South Texas Brushland (S07) and Edward's Plateau (S53) strata are largely responsible for declines in Texas. Declines also occur in the Central BBS Region and survey-wide (Trend List). The only increase is evident in New Mexico. Small sample sizes preclude analyses of the 1966-1979 trends for most states and strata, although the regional trends are generally non-significant increases during this interval. The 1980-1994 trends are generally positive, including increases in the Central and Western BBS regions and survey-wide. The trend map shows declines concentrated in the center of the breeding range from Colorado to southern Texas (Trend Map). Increases predominate along the periphery of the range.
The survey-wide indices exhibit a definite peak during 1974, followed by a decline through 1981 and then fairly stable numbers despite annual fluctuations in abundance (Survey-wide Annual Indices). Indices for the Central BBS Region and Texas are similar to the survey-wide indices. Cassin's Sparrows peaked in Colorado during 1974, followed by a decline through the mid-1980s (Colorado Annual Indices). The indices for New Mexico are quite variable, but have a general increasing tendency (New Mexico Annual Indices).
The BBS trends are influenced by the unusual abundance of Cassin's Sparrows during 1974, followed by a gradual decline to stable levels. Such short-term fluctuations are not unexpected for this species, but should not be confused with any long-term patterns in their population trends.
Wintering Cassin's Sparrows are inconspicuous inhabitants of grasslands with scattered brush in the southwestern U.S. and adjacent Mexico. They skulk through these habitats, and can be observed only with considerable effort. Hence, they are often overlooked on CBCs, being recorded in small numbers throughout the winter range. Trend estimates based on these limited data should be viewed with caution.
Andrews, R., and R. Righter. 1992. Colorado birds. Denver Mus.
Natur. Hist., Denver, CO. 442 pp.
Oberholser, H.C. 1974. The bird life of Texas, Vol. 2. Univ. of
Texas Press, Austin, TX. 537 pp.
Phillips, A.R. 1944. Status of Cassin's Sparrow in Arizona. Auk
61: 409-412.