The Ecological and Toxicological Significance of Altitudinal Migration by the American Dipper (Cinclus Mexicanus)

American dippers (Cinclus mexicanus) appear to concentrate at low elevation streams in fall and winter, implying some degree of migratory behavior in this species. Knowledge about the composition, origin and movement patterns of dipper populations are necessary to use this species as an effective biomonitor of trace contaminants in watersheds. During 1999- 2002, I color banded 522 dippers and radio-tagged 14 in the Chilliwack River watershed, British Columbia, to follow their movements during the breeding and non-breeding seasons. During the annual cycle, most individuals (84-89 %) moved from the main river to higher elevation tributaries to breed, while small numbers (11 - 16%) remained resident on the main river year round. Resident birds on the river initiated nests earlier, had greater proportions of second broods, and benefited from marginally higher rates of nest success and annual productivity than migrants on creeks. Later initiation of breeding by altitudinal migrants reduced productivity by increasing predation or flooding risk and prevented opportunities for initiating second clutches. Residue analysis of dipper eggs and prey items (benthic invertebrates and salmon fry) revealed DDE, hexachlorobenzene, trans-nonachlor and PCBs as the most prevalent organic contaminants detected. PCB and organochlorine patterns were highly consistent across sites in the Georgia Basin and between dipper eggs and prey, indicating common atmospheric sources to the region that were biomagnified in lotic food chains. Organochlorines, PCBs and mercury were all significantly higher in eggs from river residents compared to creek migrants. Feathers of residents also had higher concentrations of mercury, cadmium, and copper relative to migrants. However, levels of organic and metal contaminants in invertebrates differed minimally between the river and creeks. Observed differences in contaminant concentrations were primarily related to the proportion of salmon fry in dipper diets. Stable isotope analysis revealed that residents consumed significantly more fish than migrants. Blood [lowercase]delta [superscript]15 N values were positively correlated with total organochlorines and total DDT in eggs suggesting that fish were the primary source of organic contamination. However, daily exposure to metals that exceeded tolerable daily intake levels (selenium, aluminum and zinc) was of greater concern to migrants consuming primarily invertebrate diets.