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Reproduction of cavity‐nesting birds in pesticide‐sprayed apple orchards in southern Ontario, Canada, 1988–1994

Egg fertility, clutch size andeggandchick survival and pesticideexposureof tree swallows (Tachycineta bicolor) and eastern bluebirds (Sialia sialis) were measured annually using nest boxes in sprayed and nonsprayed apple orchards in southern Ontario, Canada, during 1988–1994. Associations were examined between reproductive rates and historical organochlorine residues in eggs as well as the degree of exposure and toxicity of pesticides applied during the study period. Because many pesticides in current use are not persistent in wildlife tissues, a toxicity score was developed to describe the exposure for each nest. The toxicity score was calculated as the product of the extent of the orchard sprayed and the application rate of the chemicals, divided by an acute reproductive toxicity index of each chemical. Total organochlorine concentrations in tree swallow eggs were 0.74 to 3.47 μg/g, and in eastern bluebird eggs, these values ranged from 0.47 to 106.3 μg/g. More than 90% of the residue in eggs was pp′DDE. There was a significant increase in unhatched eggs in bluebirds as organochlorine levels increased in eggs. There were significant associations between toxicity scores of current‐use pesticides and at least one avian reproductive parameter in every year of the study, but the reduction in reproductive rates associated with pesticides did not exceed 14%, for either species, in any year. Reduced reproduction occurred in 6 years in tree swallows but for bluebirds, this occurred in only 4 years.