Response of breeding birds to mosquito control treatments of wetlands

We examined the possible effects of mosquito control treatments of wetlands with Bacillus thuringiensis israelensis (Bti, applied as Vectobac-G granules) and methoprene (applied as Altosid sand granules) on wetland breeding bird communities. Data collected two years before (1988 and 1990) were compared to data collected three years after (1991-1993) treatments were applied. Total numbers of species and individuals observed remained relatively constant throughout the study period, but several individual species varied annually, most likely due to changes in water levels and habitat available. We found no effect of Bti or methoprene treatments on the bird community or on 19 individual bird species. The few differences that were observed between control and treatment were inconsistent over time and were likely due to chance because of the large number of comparisons that were completed. Despite relatively large reductions of aquatic insects (including mosquitoes) in mid- to late-summer following both treatment types, it is unlikely that food available to bird species in these wetlands was depressed during the breeding season. Effects of weather and predation were probably more important influences on species and community parameters than was mosquito control treatment during the study period. Because of lower aquatic insect densities in midto late-summer, other parts of the avian life cycle such as late summer survival, dispersal of young' birds, or migrating birds may be more affected by mosquito control treatments.