AMERICAN DIPPERS NESTING NEAR JUNEAU, ALASKA 
relatively few gauges measuring the great local variation of precipitation in 
the mountainous terrain around Juneau and the data available do not include 
the most recent years. In addition, use of the average elevation fails to reflect 
marked (but unrecorded) altitudinal differences in precipitation. Our index 
of stream flow was the 90% exceedance equation for the summer season, 
meaning that stream flow would exceed the calculated value 90% of the 
time. Use of the 90% exceedance equation seemed appropriate in view of 
our initial impression that some streams were “too small” to support nesting 
dippers. The estimates from other exceedance equations (e.g., 50%) were 
correlated with the one we chose, so our choice of 90% should not affect 
interpretation of results. 
Dipper Territories and Nest Sites 
We surveyed local streams for nesting activity of dippers between 05:00 
and 20:00 from late April through early August, 2004-2008, with some 
preliminary data from 2003. Initially, we surveyed each stream at least three 
times each season, from tidewater up to a point determined by hiking time 
on the nearest trail (see below). Surveys in 2004 and 2005 clearly showed 
that dippers did not nest on certain streams, and these small streams were 
not surveyed regularly in the following years. We found most nests by fol- 
lowing the birds as they carried nest material to a nest site and monitored 
these sites about twice a week until chicks fledged or the nest failed (Willson 
and Hocker 2008b). To facilitate frequent monitoring of as many nests as 
possible, we monitored only nests that were within a 3- or 4-hour round-trip 
hike of a road. A catalog of all nest locations, resident pairs, and nest success 
has been placed in the library of the University of Alaska-Southeast, Juneau. 
Because we surveyed each stream for 5 or 6 years, we knew not only the 
nest sites used each year but also the distribution of territories along the 
stream. Therefore, by comparing the distribution of territories among years, 
we could determine if a given territory was occupied in a given year. 
Dippers typically place their domed nests in protected sites very close 
to the streams where they forage (e.g., Kingery 1996). We characterized 
nest sites used each year by nesting substrate and our estimates of levels of 
protection. Each nest site was ranked in one of four levels (from 0 to 3) in 
four categories: estimated protection from flood (giving the highest rank to 
nests at a height of at least 2 m above usual summer water levels), weather 
(presence and completeness of overhang above nest), terrestrial predators 
(inaccessibility to mink, weasels, squirrels, etc., climbing along extended 
ledges or over boulders), and aerial predators (inaccessibility and lack of 
perch sites for ravens, etc.). These estimates cannot account for the effects 
of extreme events such as rare massive floods and landslides. Ranks in the 
four categories were summed for an overall estimate of level of protection 
(maximum =12). 
We also measured several features of the core of each territory at 50-m 
intervals along a 400-m reach centered on the nest site in 2004, 2005, and 
2006 for five intervals per territory. These features included stream substrate 
(visually estimated proportion of substrate occupied by bedrock, boulders 
(diameter >25 cm); cobbles (5-25 cm), gravel (2 mm-5 cm), sand, or mud, 
channel width (measured with tape or range finder), and gradient. Coarse 
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