Summer Rainfall and Winter Sparrow Densities: A Test of the Food Limitation Hypothesis

Abstract

Published data on winter sparrow densities (from Christmas bird counts [National Audubon Society, USA]) and summer precipitation were used to test the hypothesis that local densities of wintering sparrows in southeastern Arizona are affected by the abundance of food. It is assumed, that the abundance of seeds available to wintering granivorous birds is correlated with the quantity of rainfall the previous summer. The results strongly supported 2 a priori predictions: the density of sparrows should be positively correlated with summer rainfall; and the density of individual sparrow species should be positively correlated with the total density of all other sparrow species. The findings provide further evidence that winter finch populations are food-limited and suggest that interspecific competition may be important in at least some avian communities.