DISTRIBUTION, ABUNDANCE, AND SURVIVAL OF NESTING AMERICAN DIPPERS NEAR JUNEAU, ALASKA

We studied the distribution of the American Dipper (Cinclus mexicanus) near Juneau, Alaska, from 2004 through 2008. An upper limit on the local abundance and distribution of dippers in our area resulted from several factors, including stream size and food abundance, nest sites, and territorial aggression. Dippers nested only along streams with an estimated fl ow in summer of at least 0.5 cubic feet per second and nested commonly only where fl ow exceeded 0.9 cubic feet per second. Large streams provided a greater average density and estimated total abundance of benthic macroinvertebrates. Although most territories were centered on typical fast, rocky reaches of fairly high gradient, a few were centered on low-gradient reaches with a sandy substrate where anthropogenic nest sites were available. Some nests were located along glacial streams, but no nests were located along streams originating in bogs. Nest sites were typically in locations protected from predators, fl oods, and other hazards. After each of three cold winters apparent survival was low, markedly reducing the number of occupied territories; survival analysis with the program MARK showed that apparent survival decreased with decreasing winter temperature. We suggest that if dippers are used as indicators of stream quality in our area, the research should either include multi-year and region-wide surveys of distribution and abundance to account for annual variation in survival or focus on the effects of stream pollution on dipper physiology and reproduction. The American Dipper (Cinclus mexicanus) is North America’s only aquatic songbird. It nests along streams in the mountainous regions of western North America from northern Alaska to Central America and feeds on aquatic invertebrates and small fi sh (e.g., Kingery 1996, Willson and Hocker 2008b). The distribution and abundance of the dipper are often considered to be limited mainly by the availability of suitable nest sites or sometimes also by food supply (e.g., Kingery 1996). Here we consider the roles of these and other factors in limiting the distribution and abundance of dippers near Juneau, southeastern Alaska. We compare the presence of nesting dippers with estimates of stream size and prey abundance and characterize the core portions of nesting territories and specifi c nest sites. We then report variation in annual survival and its effect on territory and stream occupancy. The fi ve species of dippers can be used as indicators of stream quality because their abundance or reproductive success often decreases in response to acidifi cation, sedimentation, or industrial pollutants (e.g., Tyler and Ormerod 1994, Price and Bock 1983). Near Juneau, streams are subject to several kinds of natural and anthropogenic disturbances (e.g., Redman 1988, Swanson et al. 1998), such as mining, logging, and road building, that can affect their quality. Therefore, a baseline understanding of dipper distribution and abundance is useful for understanding and assessing future patterns. We here provide a baseline for future comparisons of the distribuAMERICAN DIPPERS NESTING NEAR JUNEAU, ALASKA 192 tion and abundance of the dipper near Juneau and discuss the species’ use as an indicator of stream quality.