Size and Structure of a Wintering Avian Community in Southern Texas

Abstract

The wintering land birds of a 25.4—km2 area centered at the Welder Wildlife Refuge is southern Texas ware subjected to systematic counting, and the counts were converted to absolute density values by applying an empirically derived conversion factor correcting for variable detectability and reducing the adjusted values to a common areal base. The vegetation was mapped, and seven vegetation types–two of grassland, three of brushland, and two of woodland and riverine forest–were recognized and defined in terms of measurements along prominent parameters of the vegetational physiognomy. The combined density for the 50 species constituting the land bird community was 918 individuals and 96 kg of avian biomass per km2 (371 individuals and 85 lb. per 100 acres). Discounting the heavy—bodied Wild Turkeys and Bobwhite Quail, there were 729 individuals and 33 kg per km2. Winter invaders outnumbered permanent residents in terms of species but had fewer individuals and a smaller biomass. Without the quail and turkey, there were more individual granivores than insectivores, but a smaller granivore biomass, both in the winter invader and the permanent resident categories. Winter invader granivores were primarily small—seed foragers. The number of species and the species diversity (H') were higher in the habitats containing trees and high shrubs than in open prairies; brushlands held an intermediate position. Densities and biomass were high in the wooded and open habitats and low in the brushlands. A measure of equitability reflecting the steepness of the species abundance curve in each habitat showed low values (steep curves) in the forest and scrubby prairie habitats. Winter invaders were relatively best represented in the woodlands and most poorly represented in the brushlands. In absolute numbers, however, they were highest in the forest. Insectivores were better represented in the brushlands than in either the prairies of wooded habitats. Each bird species showed a distinctive pattern of distribution through the seven habitats; some were highly eutytopic, appearing in all habitats; others were restricted to one or to. The Bobwhite Quail dominated (highest density) in all vegetation types except the forest were the Cardinal was a strong dominant. The top four species in each type comprised from 55% to 80% of the combined population of the type. An index of ecological amplitude gave values ranging from .00 in the Wild Turkey of the woodlands to .96 in the ubiquitous Robin. No correlations of ecological amplitude with migratory status or food habits were detected. An index of ecological overlap was derived for each two—species combination by summing the differences in representation of the two in the seven habitats and dividing this total by the maximum possible score achieved if the two species had identical ecological distributions. Values ranged from 1% (White—throated Sparrow x Savannah Sparrow) to 94% (Hermit Thrush x Ruby—crowned Kinglet). The amount of overlaps in the between—category dyads (summer—winter) was intermediate between the overlaps in the two intracategory dyads (summer—summer and winter—winter). A two—dimensional ordination depicting the intrinsic ecological distinctness of the commoner species in the community shows three clusters of points and a single isolate. The three clusters correspond to grassland, forest, and forest—brushland groups; the isolate is a stenotopic brushland species. Both permanent residents and winter invaders were well represented in each group.