Wintering Ecology of Thrashers in Southern Texas

-Three sympatric thrashers, the Brown, the Long-billed, and the Curve-billed (Toxostoma spp.), were studied during the winters of 1977 and 1978 in San Patricio County, Texas. The species avoided competition primarily by occupying different habitats. Brown Thrashers wintered abundantly within riparian woodlands, Long-billed Thrashers stayed mostly within shrub cover of chaparral, and Curve-billed Thrashers inhabited the more open portions of the chaparral. All three species foraged mostly on the ground and were omnivorous. The foraging behavior of Brown and Long-billed thrashers was similar (sweeping debris aside with bill) and both fed only within cover. Curve-billed Thrashers differed from the other thrashers in foraging techniques (reliance upon digging), diet, and in feeding frequently outside of shrub/tree cover. Brown and Long-billed thrashers maintained intraand inter-specific winter territories. Both were highly philopatric, with 27% of the color-marked Brown Thrashers and 48% of the color-marked Long-billed Thrashers returning in 1978 to their 1977 winter territories. Brown Thrashers apparently were superior competitors, mostly excluding Long-billed Thrashers from arthropod/gastropod-rich riparian habitats. Competition may be severe between sympatric congeners. As a result of such interactions, congeners tend to replace each other abruptly between habitats or geographic regions (Lack 1971). Thrashers of the genus Toxostoma exemplify such segregation, with usually only one species occurring within a geographically distinct, homogeneous type of vegetation (Cody 1974). Southern Texas presents an unusual situation, though, since three thrashers, the Brown Thrasher (T. rufum), the Longbilled Thrasher (T. longirostre), and the Curve-billed Thrasher (T. curvirostre), are sympatric during the winter. Each of these species has broadly similar ecological needs, and all are basically omnivorous and terrestrial (Bent 1948). Curve-billed and Long-billed thrashers remain in southern Texas throughout the year, and breed in different habitats (Fischer 1980). The purpose of this study was to investigate habitat selection, behavioral interactions, foraging behavior, and food habits of wintering thrashers in order to determine their means of ecological segregation. STUDY AREAS AND METHODS My study was conducted from September through April 1977-1978 on the Rob and Bessie Welder Wildlife Foundation (hereafter Refuge), San Patricio County, Texas. Three study plots, 3.35 ha each, were selected: two within riparian woodland and one within dense brushland (chaparral). The vegetation of each plot was analyzed in September 1977 with 25 30.5 m-line transects (Canfield 1941). Botanical nomenclature follows Jones (1975). The two riparian plots, Plots I and II, supported a similar vegetational cover: 209.5% and 208.8% (overlapping foliar layers), respectively. They differed greatly, however, in size, density, and species composition of the trees. Plot I was characterized by large trees, primarily sugar hackberry (Celtis laevigata), netveined hackberry (C. reticulata), anacua (Ehretia anacua), and elm (Ulmus crassiflora) that formed a dense, closed canopy. The understory cover was light, 28.8%, and comprised mostly of colima (Zanthoxylum fagara). Grass and forbs formed 36.2% of the total cover. The tree cover of Plot II was composed mostly of small, densely packed sugar and net-veined hackberry, la coma (Bumelia celastrina), and Texas persimmon (Diospyros texana). Anacua and elm were virtually absent. The understory was poorly developed, with shrubs contributing 7.6% of the cover. Grass-forb cover, 46.8%, was slightly greater than that of Plot I. In the chaparral plot, Plot III, mesquite (Prosopis glandulosa) and huisache (Acacia farnesiana) were the only tree species recorded, although their cover was slight (8.6%). Shrubs comprised about 60% of the total cover, 168.7%, and the remainder consisted of grasses and forbs. The shrubs grew in dense, mixed thickets ("mottes") composed mostly of blackbrush acacia (A. rigidula), agarito (Berberis trifoliata), brasil (Condalia hookeri), colima, and granjeno (Celtis pallida). I mist-netted in Plot II and Plot III in 1977-1978 and Plot I in 1978 to determine the habitat selection and site fidelity of the thrashers. Each thrasher captured was banded and marked with an individually color-coded leg-streamer. In addition, I censused thrasher populations by walking rapidly through each plot at dawn or dusk and recording the number of calling birds. Both Brown and Long-billed thrashers called vigorously for approximately 15 min during early mo ning and late evening. I compa d 100 foraging sites of Long-billed and Curv -billed thrashers in Plot III by measuring the diameter of the mottes in which they were foraging, and by measuring the distance to the nearest adjacent motte. Foraging behavior of 12 Brown and 7 Longbilled thrashers was quantified following Cody (1974).