Effect of altitudinal migration within a watershed on the reproductive success of American dippers

Distinct changes occur in the distribution of American dipper (Cinclus mexicanus Swainson, 1827) populations during the breeding season. Small numbers of American dippers remain resident on the wintering site, while the majority of birds make short altitudinal movements upstream. Therefore, American dippers breed over large elevation gradients within a watershed, using both the main river and its associated tributaries. I hypothesized that altitudinal migration of American dippers would affect their timing of breeding and ultimately their productivity. Additionally, since the main river and its tributaries differ in habitat, elevation, and nesting substrates, I hypothesized that these variables would also influence American dipper breeding performance. In the Chilliwack watershed of British Columbia, Canada, I followed 99 pairs of American dippers: 23 in 1999, 40 in 2000, and 36 in 2001, of which approximately 65% were residents and 35% were migrants. Resident pairs on the lower elevation river initiated nests earlier and a greater proportion had second broods, contributing to slightly higher nest success and annual productivity compared with migrants on tributaries. Reduced productivity was primarily associated with later onset of breeding, which increased the likelihood that nests were lost to predation or flooding and reduced the probability of initiating a second clutch. Timing of breeding was affected by migratory status and year, but elevation and habitat did not directly influence breeding perfor-