DISTRIBUTION, ABUNDANCE, AND SURVIVAL 
OF NESTING AMERICAN DIPPERS 
NEAR JUNEAU, ALASKA 
MARY F. WILLSON, 5230 Terrace Place, Juneau, Alaska 99801; mwillson@gci.net 
GREY W. PENDLETON, Alaska Dept, of Fish and Game, 802 3 rd Street, Douglas, 
Alaska 99824 
KATHERINE M. HOCKER, 7995 North Douglas Highway, Juneau, Alaska 99801 
ABSTRACT: We studied the distribution of the American Dipper ( Cinclus mexi- 
canus ) 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, includ- 
ing 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. 
The American Dipper ( Cinclus mexicanus ) is North America’s only 
aquatic songbird. It nests along streams in the mountainous regions of west- 
ern North America from northern Alaska to Central America and feeds on 
aquatic invertebrates and small fish (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 specific nest sites. We then report variation 
in annual survival and its effect on territory and stream occupancy. 
The five species of dippers can be used as indicators of stream quality 
because their abundance or reproductive success often decreases in response 
to acidification, 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 distribu- 
Western Birds 40:191-209, 2009 
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