Linking contaminant profiles to the diet and breeding location of American dippers using stable isotopes

1. Individual variation in contaminant levels is a common occurrence in many toxicology studies, but the exact cause is often unclear. We investigated the influence of diet and breeding location on individual bird contaminant profiles using a colour-marked population of American dippers Cinclus mexicanus. The population comprised two distinct groups within a single river system: resident dippers breeding on the main river and altitudinal migrants breeding on tributaries. 2. Residue analysis revealed that total organochlorines (OC), polychlorinated biphenyls (PCB) and mercury (Hg) were all significantly higher in eggs from river residents compared with tributary migrants. This trend was also apparent for the three most prevalent organochlorine compounds: p,p'-dichlorodiphenyl-dichloroethylene (DDE), hexachlorobenzene and trans-nonachlor. 3. We hypothesized that the observed differences in contaminant concentrations were partially related to the proportion of salmon fry Oncorhynchus spp. in the diet relative to aquatic invertebrates. Stable isotope analyses using delta(13)C and delta(15)N were conducted on blood and feathers of dippers in addition to aquatic invertebrates and salmon fry prey. Linear mixing models using the N-15 isotope in the dippers' diet and blood revealed considerable variability in the proportion of fish consumed (0-71%). Resident dippers on the main river ate significantly higher proportions of fish (42%) than tributary migrants (22%) (P = 0.01). 4. The difference in diet between migratory groups explained some of the observed variation in egg contaminant profiles, as total OC (P = 0.002) in dipper eggs was positively correlated with blood delta(15)N values, indicating fish may be the primary source of contamination. 5. Synthesis and applications. We conclude that dipper eggs represent local conditions at the breeding site, making them useful tools for biomonitoring aquatic contaminants in watersheds. However, given the distinct difference in contaminant profiles between resident and migrant dippers and the link with diet, the results emphasize the importance of understanding individual species' ecology for assessing toxicological effects at the population level.