Population Structure in Grassland Bird Communities during Winter

-Mid-winter populations of birds at 20 open-grassland sites of varied grazing pressure and cultivation practices in Oklahoma and Texas were examined to evaluate patterns of avian species’ abundances and distributions. Most of the 23 non-raptorial bird species observed were winter residents. Of these, only 14 were present on more than 2 of 53 censuses. Except for meadowlarks and the Sprague’s Pipit (Anthus spragtleiz), these species are granivores. Regional distinctions in species composition of 5”-latitude-longitude blocks occur along northsouth and east-west axes in the south-central United States. A cluster analysis of sites grouped them primarily on the basis of the most abundant bird species. Sites with similar grazing pressures were generally placed into the same cluster. However, sites in Oklahoma classified as moderately grazed were grouped with either heavily or lightly grazed grasslands depending on the predominant avian species. Estimates of bird biomass were highest on northern Oklahoma and southern Texas sites. Biomass was higher on heavily grazed than lightly grazed grasslands in Oklahoma and western Texas. In contrast, moderately grazed grasslands in southern Texas supported significantly more bird biomass than heavily grazed sites. Total granivore biomass was correlated with seed abundance (r = 0.78). Annual changes in bird biomass were generally consistent among sites (and also among species) in central Oklahoma. The year 1976 was a drought year and fewer birds were present during the winter of 1976-l 977 than in other winters. However, Savannah Sparrows (Passerculus sandwichensis) were less numerous and Smith’s Longspurs (Calcariuspictus) most abundant during the coldest winter (1978-l 979) with unusually high snowfall. The highest winter bird population estimates in southern Texas occurred in a period with record rainfall (1976-l 977). Population dynamics and the role of birds as consumers in grassland ecosystems have been studied intensively during the breeding season (Mitchell 196 1, Finzel 1964, Spiers and Orenstein 1967, Feist 1968, Owens and Myres 1973, Wiens 1973, 1974a, b, Raitt and Pimm 1976). Analyses of grassland bird populations at other seasons, however, are limited in scope (Quay 1947, Finzel 1964, Emlen 1972, Raitt and Pimm 1976) some being restricted to counts of birds from roadway routes (Wiens et al. 1972, 1974, Risser 198 1). As indicated by Wiens (1974a) and Wiens and Dyer (1975), analysis of grassland bird communities is hampered by the paucity of quantitative population studies conducted during non-breeding periods. Yet, much of the structure of bird communities during the breeding season may be determined by winter resources (Pulliam and Enders 197 1, Fretwell 1972, Wiens 1974a, Raitt and Pimm 1976) and by climatic factors influencing birds during the winter. In temperate grasslands, the winter differs from the breeding season in several important ways for grassland birds. Many species occupy different geographical ranges, are subjected to different climatic conditions (often more severe in winter), and use different foods. The food for many temperate passerine species during winter consists primarily of seeds, which can be abundant (Johnston and Odum 1956); seed abundances are established at the time of the first frost, and decrease as the season progresses. The days are shorter in winter and afford less time for all activities. Competition may be heightened by increased over-