Productivity and survival of Great Horned Owls exposed to dieldrin

We investigated the influence of dieldrin, a persistent organochlorine, on a top trophic level raptor, by comparing productivity and survival of Great Horned Owls (Bubo virginianus) using contaminated and uncontaminated areas . Between 1994-1996, we worked on a Superfund hazardous waste site, the Roc ky Mountain Arsenal National Wildlife Refuge in Colorado. Productivity of o wls did not differ between those using the contaminated and uncontaminated areas. Adults in the uncontaminated area had nominally higher survival from 15 January-15 August. Mortalities related to dieldrin in adults were most likely the result of mobilization of dieldrin from other tissues due to nut ritional stress associated with reproduction. Interval survival of juvenile s in contaminated and uncontaminated areas was similar, however, dieldrin r esidue in plasma at nine weeks of age was strongly and negatively related t o survival; juveniles with higher dieldrin residues died during dispersal. Plasma dieldrin concentrations in juveniles correlated with dieldrin levels in soil and prey within parental home ranges. Consequently, sampling dield rin in juvenile plasma is a more reliable method for monitoring the impact of dieldrin than counting number of young fledged per nest.