Michael W. Monahan, Department of Biological Sciences, University of
Denver, Denver, CO 80208
Steven T. Faulk, Foster Wheeler Environmental, Lakewood, CO
80228
This study explored the factors shaping avian use of 36-closed canopy, cattail wetlands on the Colorado Piedmont. Six species (sora and Virginia rail, red-winged and yellow-headed blackbird, yellowthroat, and song sparrow) were censused on three dates by one of three methods, song playback for rails, visual scanning for blackbirds, and line transects for yellowthroats and song sparrows. The presence and breeding status of other bird species (principally waterfowl species but also mourning dove, common grackle, and American coot) were recorded while censusing the six "target" species. Wetland shape, size and edge density were determined from aerial photographs. Transect sampling was used to quantify hydrologic and vegetation features (floristics and physiognomy) of each wetland. Objectives of the study were to identify habitat correlates of avian species richness and densities of individual wetland residents.
Breeding bird richness on individual wetlands varied from 1 to 13 with a modal value of 5 to 6 species. In general, the species makeup of individual communities was a function of community size. Thus, redwings were the only species to appear alone, yellowthroats typically joining redwings in two-species communities, song sparrows or Virginia rails being the third member of 3-species communities, and so on. In effect, the species tended to form a "nested series," implying that the resource needs of the habitat specialists were subsumed under those of the habitat generalists. Soras and yellow-headed blackbirds were uncommon members of these communities (12 and 4 sites, respectively) and only occurred on sites that also had other members of the breeding bird community.
Roughly half the variance (R2 of 52%) in avian species richness across sites was accounted for variation in wetland size (range, 0.5 to 5.0 ha). Using stepwise regression analysis, within site diversity in water depth accounted for the next largest fraction of the variance in avian species richness (R2 of 73% for the two-variable model). The various measures of plant floristics and physiognomy showed little or no associations with avian species richness after wetland size and hydrology were taken into account. Likewise, wetland shape, as measured by edge length relative to wetland area, had little or no measurable effect on avian species richness. Results were similar when wetlands were compared for variation in richness of individual bird taxons (6 species of songbirds, 3 species of rails, and 5 species of waterfowl.
Variation in wetland size was the single strongest correlate of resident population size for song sparrows (R2 of 36%), red-winged blackbirds (R2 of 49%), Virginia rails (R2 of 49%), and yellowthroats (R2 of 68%). Sora numbers correlated most strongly with area of wetland containing standing water (R2 of 64%). When subjected to stepwise regression analysis, models incorporating wetland size and hydrology accounted for most of the explained variation in population sizes of these 6 resident species.
For closed canopy, cattail wetlands on the Colorado Piedmont, avian species richness in the breeding season is likely to be highest on sites that are large, spatially diverse in depth of standing water, and settled by soras and/or yellow-headed blackbirds. Soras and Virginia rails were the only two wetland specialists among webless species in these breeding bird communities. For purposes of estimating resident population size of these two rail species, the present results point to area of measurable water as a reliable indicator.