AMERICAN DIPPERS NESTING NEAR JUNEAU, ALASKA 
Invertebrate densities (invertebrates/sample) 
Figure 3. Density of macroinvertebrate prey per sample in reaches of streams 
occupied and unoccupied by the American Dipper near Juneau, Alaska. 
Of 64 nest sites used in at least one year, 58% were on rocky cliffs, 19% 
were in cavities in boulder piles, 6% were in old wooden dams, 6% were on 
bridges, and the remainder were in other categories (i.e. , a log on a retain- 
ing wall, boulder top, steep shale slope, tree-root cavity, concrete spillway). 
Eleven sites (17%) were anthropogenic in origin. 
By our estimates, 75% of nest sites had a high overall degree of protection, 
with a combined score of at least 10 (of a possible 12). Four nest sites (6%) 
had little protection from weather, and six sites (9%) had little protection 
from aerial predators (Table 2). Ten sites (16%) were at some risk of flood, 
and nine (14%) had little protection from terrestrial predators. Despite the 
potential risk, most of these nests were probably successful (Table 3; see also 
Willson and Hocker 2008b). One site on the top of a boulder was exposed 
to all potential dangers but fledged two broods in 2005. 
Annual Variation in Distribution and Abundance 
American Dippers nested on up to 21 of the 33 streams we surveyed 
intensively, depending on the year. Sometimes we found vacant territories 
on stream reaches that had been occupied in previous years or were occu- 
pied in subsequent years. For 31 territories for which we have at least four 
seasons of data, the frequency of vacant territories was 13% in 2004, 19% 
in 2005 and 2006, but 48% in 2007 and 32% in 2008 (Table 3); in 2003, 
11% of known territories (n = 19) were unoccupied. 
The rate of vacancy was greatest after three years of low apparent annual 
survival (from one nesting season to the next) of banded adults (2005-2006, 
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