The Olive Sparrow is a common permanent resident of the Atlantic and Pacific lowlands of Mexico, with a range extending South to Guatemala, Belize and northwestern Costa Rica. In the United States, Olive Sparrows occur only in semiarid South Texas. Their range coincides exactly with the Tamaulipan Biotic Province or “brush country” of South Texas. Preferring dense brush or chaparral, Olive Sparrows are easy to hear but difficult to see, as they forage on the ground in shady areas. Only diligent work allows confirmation as a breeder, even though they are one of the more common species in much of South Texas (e. g., Carter 1986, Gehlbach 1987). Evidence of its sedentary nature is provided by Parkes (1974), who showed subspecific variation within 11 km (7 mi.) in the Yucatan Peninsula of Mexico.
DISTRIBUTION: As noted above, the Olive Sparrow is a Tamaulipan Biotic Province bird. Olive Sparrows inhabit dense and semiopen thornscrub (Oberholser 1974) but avoid mesquite savannah and grassland (Roth 1977). Riparian forest and oak mottes are used, as long as some shrubs occur in the understory (Brush unpub. data). The range in the United States, as revealed by the TBBAP, is essentially identical to the map in Oberholser (1974) and that produced by the BBS. The northern limit of its range runs roughly from Del Rio, East along the southern fringes of the Hill Country and Southeast to Beeville and Aransas National Wildlife Refuge, avoiding the urban/suburban San Antonio metropolitan area. At the eastern edge, Olive Sparrows avoid coastal prairie and barrier islands, but occur within 50 m of coastal flats and bays when dense brush is present (Brush, unpub. data). Christmas Bird Count data show a tendency for Olive Sparrows to be irregular at their northern limits. This may be due to small numbers and preference for the densest brush in winter, rather than migration (Brush, unpub. data). Three birds banded as adults at Laguna Atascosa National Wildlife Refuge remained in the same brushy area 3 years later (refuge records). Few vagrants have been reported: Oberholser (1974) doubted sight records for Waco and Austin, while Wauer (1985) reported a sight record, by an experienced observer, in Big Bend National Park. Both the nonmigratory status and retiring habits make vagrancy unlikely to be reported.
SEASONAL OCCURRENCE: The Olive Sparrows breeds from March through September (Oberholser 1974), as might be expected for a subtropical permanent resident. Singing occurs during that entire period, although most frequently in May and June (the height of the nesting season; Bent 1968). Rainy periods, especially in August and September, stimulate singing and egg-laying (Sennett 1878). Individuals have also been heard singing in February and October, suggesting possible breeding then. Birds may move locally on a seasonal basis (Gehlbach 1987).
BREEDING HABITAT: Dense thornscrub appears to be preferred nesting habitat for Olive Sparrows (Oberholser 1974). Singing olsppen.gif (75528 bytes)
Art by Mimi Hoppe Wolf males occur in large and small brush and narrow, field-margin brush (Brush, unpub. data), but it is not known if birds breed in narrow strips. Lower nesting densities occur in semiopen oak mottes, open scrub and successional fields. Revegetated areas >30 years old may support densities as high as in mature thornscrub (P. Wright, T. Brush and M. Bray unpub. data). A leaf litter layer may be important, since Olive Sparrows do most of their foraging on the ground and frequently scratch in the litter. The domed nest (Sennett 1879) may be placed on the ground or 1-2 m up in a woody plant. Nests are often well hidden in or under grassy or woody cover, but may also be placed in pricklypear cacti (Friedmann 1925).
STATUS: There is little information regarding population trends in Olive Sparrows. BBS data show conflicting patterns, with increases during the 1966-1979 period followed by decreases during 1980-1994. With <15 survey routes having Olive Sparrows, any trends should be treated with caution. Some declines may have occurred due to massive brush clearing following World War II (Oberholser 1974), but conversion of prairie or savannah grassland to brushland (Rhoads 1892, Johnston 1963) may to some extent counterbalance any declines. On a local basis, the density of breeding Olive Sparrows increased on a study plot in Santa Ana National Wildlife Refuge, as the area changed from Texas ebony evergreen woodland in the 1970’s (Gehlbach 1987) to Tamaulipan thornscrub by the 1990’s (Brush and Cantu, unpub. data). Both Bronzed (Molothrus aeneus) and Brown-headed Cowbirds (M. ater) parasitize Olive Sparrow nests (Friedmann and Kiff 1985). Limited data suggests that once parasitized, Olive Sparrow nests generally raise only cowbirds (Carter 1986). Since cowbirds often find nests by following adults to nests (Carter 1986), dense cover may be crucial in allowing some Olive Sparrow nests to go undetected and produce young Olive Sparrows. The species remains common in suitable thornscrub habitat in South Texas. Text by Tim Brush (ca. 1993)
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