AMERICAN DIPPERS NESTING NEAR JUNEAU, ALASKA 
restricting the number of available breeders (Sullivan 1973, Price and Bock 
1983). Nocturnal drainage of cold air down narrow ravines (see Pypker et 
al. 2007) and the high winds common around Juneau may exacerbate the 
effects of low temperature. Low nest success in the cold, wet summer of 
2006 (Willson and Hocker 2008b) probably contributed to the decline in 
abundance in 2007. 
Weather is also known to affect populations of the White-throated Dipper. 
For example, populations of this species in France fluctuated in response to 
flood and drought (Marzolin 2002). Furthermore, low winter temperatures 
contribute significantly to low overwinter survival of many birds (e.g., Nils- 
son 1987, Arcese et al. 1992, Robinson et al. 2007, Flockhart and Wiebe 
2008), including the White-throated Dipper (Saether et al. 2000, Loison et 
al. 2002; see also Tufto et al. 2000). 
The American Dipper as an Indicator Species 
Southeast Alaska is subject to natural and anthropogenic disturbances that 
affect the quality of stream water (e.g., Swanson et al. 1998), so the dipper 
could be useful here as an indicator species for stream quality. Many local 
streams that can support nesting dippers, however, are so small that only 
one pair has been found to nest along them, so the absence of a single pair 
from a small stream would not be valid evidence of impaired stream quality. 
Low overwinter survival, as we observed, could also be the cause. Using 
dipper distribution and abundance as an indicator of stream quality neces- 
sitates a multi-year, regional assessment of the population rather than simple 
before-and-after surveys of a single stream subject to suspected disturbance. 
Annual variation in overwinter survival (and nest success) has too great an 
effect on distribution and abundance. 
Therefore we suggest that a more direct way of using dippers as indicators, 
in our area, is to assess the birds’ physiological and reproductive condition. 
Studies of the White-throated Dipper have documented many effects of 
pollution, suggesting many possible responses that could be investigated in 
the American Dipper. The White-throated Dipper accumulates toxins, lives 
at lower densities, and eventually abandons streams polluted with sewage, 
industrial and agricultural waste products, and heavy metals draining from 
mines in several regions of Europe (Monig 1985, Tyler and Ormerod 1994, 
Sorace et al. 2002). Stream acidification from industrial emissions and plan- 
tations of conifers in Britain and other parts of Europe has had numerous 
detrimental effects on dippers there, including poor body condition, later 
egg-laying dates, decreased eggshell thickness and egg mass, smaller clutches 
and broods, increased time spent foraging, lower rates of food delivery to 
chicks, lower rates of energy gain, slower nestling growth, lower chick 
weights and survival, decreased frequency of second clutches, and ultimately 
lower population density (Ormerod et al. 1985, 1988, 1991, Ormerod and 
Tyler 1987, 1990, 1996, O’Halloran et al. 1990, Vickery 1991, 1992, 
Logie 1995, Logie et al. 1996, Sorace et al. 2002). Many of these effects 
were induced by changes in the abundance and taxonomic composition of 
prey (e.g., Ormerod et al. 1985, 1988, Ormerod and Tyler 1991, Vickery 
1991). Among other things, changes in the principal prey types resulted in 
lowered calcium intake (Ormerod and Tyler 1986, Ormerod et al. 1988, 
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