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

Egg fertility, clutch size and egg and chick survival and pesticide exposur e of tree swallows (Tachycineta bicolor) and eastern bluebirds (Sialia sial is) 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 applie d during the study period. Because many pesticides in current use are not p ersistent in wildlife tissues, a toxicity score was developed to describe t he 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 chemi cals, divided by an acute reproductive toxicity index of each chemical. Tot al organochlorine concentrations in tree swallow eggs were 0.74 to 3.47 mu g/g, and in eastern bluebird eggs, these values ranged from 0.47 to 106.3 m u g/g More than 90% of the residue in eggs was pp'DDE. There was a signific ant increase in unhatched eggs in bluebirds as organochlorine levels increa sed in eggs. There were significant associations between toxicity scores of current-use pesticides and at least one avian reproductive parameter in ev ery year of the study, but the reduction in reproductive rates associated w ith pesticides did nor 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.