EFFECTS OF HERBICIDE-INDUCED HABITAT ALTERATIONS ON BLACKBIRD DAMAGE TO SUNFLOWER

In August 1992, we treated cattail-dominated wetlands in four 23-km(2) blocks with aerially-applied glyphosate herbicide (5.31 hd(-1)). Four other blocks of wetlands were left untreated (reference). We assessed the effects of cattail (Typha spp.) reduction on roosting blackbird ( Icterinae) numbers and sunflower damage within the blocks. Blackbird n umbers did not differ between posttreatment years (P = 0.453) or betwe en treated and reference wetlands (P = 0.469), averaging 6227 +/- 4185 (SE) birds per block. Sunflower damage within blocks was similar betw een posttreatment years (P = 0.250) and did not vary (P = 0.460) betwe en treatments ((x) over bar = 2.9 +/- 1.2%). However, positive linear relationships were detected between blackbird numbers (y) and hectares of live cattails (x) [(y = 442.2 x)] (p = 0.006) and between blackbir d numbers (x) and kilograms of sunflower lost per hectare per year (y) [(y = 0.003 x)] (P = 0.0001). Cattail reduction appears to discourage roosting blackbirds and, thus, may reduce sunflower damage in adjacen t fields.