Population Structure in Grassland Bird Communities during Winter

Abstract

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 spragueii), these species are granivores. Regional distinctions in species composition of 5°-latitude-longitude blocks occur along north-south 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 1976-1977 than in other winters. However, Savannah Sparrows (Passerculus sandwichensis) were less numerous and Smith's Longspurs (Calcarius pictus) most abundant during the coldest winter (1978-1979) with unusually high snowfall. The highest winter bird population estimates in southern Texas occurred in a period with record rainfall (1976-1977).