Nest-site selection and productivity of American Dippers in the Oregon Coast Range

Availability of high-quality nest sites is thought to limit breeding populations of American Dippers (Cinclus mexicanus). To examine this hypothesis, we characterized dipper nest sites, nest-site habitat, and productivity in the central Oregon Coast Range. We also made additional nest sites ("created" nest sites = nest boxes, cliff ledges, hollowed logs that we constructed or created) along one of two creeks. Suitable nest sites (1) provided a physical space to place the nest, (2) were above the upper reach of flooding and inaccessible to ground predators, and (3) were very near to, or extended over, the stream's edge. Given these requirements, and within the context of swift, unpolluted mountain streams, dippers exhibited flexibility in their nest-site selection patterns and used a variety of nesting substrates. Streamside features associated with dipper nest sites included geomorphically constrained valleys (i.e., narrow valley floors), the presence of trees in the riparian zone (not tested statistically, but nearly universal to all nest sites), stream shading from overhead vegetation, and locations that were farther from areas frequented by humans (e.g., roads). Dippers readily used nesting substrates that we created, more than doubling the breeding population on a 10-km reach of stream (8 versus 3 nests/10-km reach). Reproductive success was high and not associated with any habitat feature we measured. The factors influencing recruitment in the Oregon Coast Range remain unknown.