Scaling of Host and Environmental Patchiness

The species distributions should be constrained by spatially extensive variables such as climate, by habitat distribution, and by spatial patchiness, microhabitat attributes, and species interactions. Brood parasites might be expected to depend as strongly on host distribution as on biophysical factors. Classification and regression trees (CART) analysis of Breeding Bird Survey (BBS) and Christmas Bird Count (CBC) data were used to derive hierarchically organized statistical models of the influence of climate and weather, cropping and land use variables, and host distribution on the distribution of the Brown-headed Cowbird (Molothrus ater) within the conterminous United States. Over the period 1972-89 the abundance of cowbirds was correlated with cropping variables (median 15.5%), weather and climate (13.3%), and region (9.6%); and a further 11.3% was associated with Farm Conservation Reserve Program (CRP) enrollments once that program began. In contrast, a single year's (1985) model of the CBC distribution was most strongly correlated with climate variables (26.5%) and more weakly with crop practices (7.4%) and region (5.9%). Overall the species richness component contributed 39% of the predictive power of the model. These results suggest that the distribution of hosts takes precedence over habitat attributes in shaping cowbird distribution, within an envelope of constraint set by biophysical factors.