Numerical responses of red-winged blackbird populations to changes in regional land-use patterns

Abstract

The red-winged blackbird (Agelaius phoeniceus) population in southwestern Quebec more than doubled between 1966 and 1981, apparently in response to increased corn production. In southwestern Ontario less pronounced increases in blackbirds and corn production also occurred. Although larger blackbird populations have been responsible for greater crop damage, it is the waste corn left in fields by mechanical harvesters that supports the increased blackbird population. The increased availability of waste corn during the spring and summer (breeding season) was likely most important in allowing the blackbird population to grow. This conclusion is supported by our census of blackbird populations in 38 counties in southwestern Quebec during the breeding season, which indicated a strong positive correlation between blackbird abundance and corn production in a county. Although corn production explains much variation in the size of blackbird populations, other factors such as the availability of breeding habitat and the spatial distribution of habitats (landscape heterogeneity) may be responsible for additional variation.