The effect of fine-scale variations in agricultural land use on the abundance of red-winged blackbirds

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Abstract
We used censuses of breeding red-winged blackbirds (Agelaius phoeniceus) and of land use to test the prediction that blackbirds should be most abundant where there is a mix of hayfield (breeding habitat) and corn production (feeding habitat) but fewer birds where either habitat predominates. Red-winged blackbird abundance increased in response to increases in both hayfields and cropland. Multivariate analysis including hayfield, cropland, and wetland abundance as explanatory variables explained substantially more variation in blackbird abundance than any of the variables could by themselves. Although our analyses indicated some evidence of a decline in numbers of blackbirds when either hayfields or cropland became very abundant, none of the areas censused approached true monoculture and therefore the conditions under which a sharp decline in the blackbird population was predicted were not encountered. However, evidence from other locations and local population trends suggest that the prediction is correct.