eWestern Birds

Distribution, Abundance, and Survival of Nesting American Dippers Near Juneau, Alaska

Mary F. Willson, Grey W. Pendleton, and Katherine M. Hocker

ABSTRACT: 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 flow in summer of at least 0.5 cubic feet per second and nested commonly only where flow 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, floods, 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.

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