AMERICAN DIPPERS NESTING NEAR JUNEAU, ALASKA 
Table 1 Characteristics of Streams along 
400 m Centered on Known Nest Sites 0 of 
the American Dipper near Juneau, Alaska 
Characteristic 
n 
% 
Channel width fa 
2-6 m 
19 
37 
6-10 m 
14 
26 
>10 m 
19 
37 
Stream substrate 0 
Bedrock and boulders 
32 
62 
Cobble 
16 
31 
Sand and silt 
4 
7 
Gradient 0 
High 
29 
56 
Medium 
16 
31 
Low 
7 
13 
a n = 52. 
b Categories are arbitrary. 
c Categories defined under Methods. 
2006-2007, 2007-2008) and one season of lower nest success (Table 3). 
Although apparent adult survival was low in the winter of 2005-2006, nest 
success the previous season was high (Table 3), so territory vacancy was rela- 
tively low. The winter of 2006-2007 had low adult survival and was preceded 
by a season of low nest success, and territory vacancy in 2007 was high. The 
winter of 2007-2008 had low adult survival; it was preceded by a year of 
relatively good nest success but few breeding pairs, so recruitment was low 
and in 2008 many territories were vacant. Several small streams previously 
used for nesting had no nesting dippers at all in 2007 and 2008. 
We found strong evidence for annual variation in survival rates; the only 
model that did not allow survival to vary by year but only by sex had a weight 
of <0.01 (Table 4). The best model, with ~2.7 times the support of the 
Table 2 Numbers of American Dipper Nest Sites 0 
at Four Estimated Levels of Protection from Four Risk 
Factors 
Estimated level of protection 
Risk factor fa 
High 
Medium 
Low 
None 
Weather 
52 
8 
2 
2 
Flood 
35 
19 
10 
0 
Aerial predators 
50 
8 
4 
2 
Terrestrial predators 
35 
20 
8 
1 
a n = 64. 
fa Ranked from zero to high and summed for a combined estimate. 
199 
