AMERICAN DIPPERS NESTING NEAR JUNEAU, ALASKA 
Many small streams in our study area originate in muskeg bogs at mid to 
low elevations but were unoccupied by dippers, despite the occurrence of 
apparently suitable nest sites on some of these streams. Possible explana- 
tions for the lack of nesting dippers on streams originating from bogs might 
include the lack of prolonged input of snow melt from high elevations and 
characteristics of the streams’ chemistry (David D’Amore, Richard T. Ed- 
wards, Forestry Sciences Lab, Juneau, pers. comm.). Although acidification 
of streams in Britain has led to low density of dippers and their prey (e.g., 
Tyler and Ormerod 1994), all of the streams we sampled, with pH between 
5 and 6, are at least as acidic as acidified streams in Britain. Perhaps the 
aquatic invertebrates in western North American streams are better adapted 
to low pH than the European species. We cannot determine if the density 
and reproductive success of American Dippers (and their prey) are lower 
than they would if our streams were less acidic; a comparison with streams 
in Alaskan karst landscapes might be informative. 
The broad overlap between occupied and unoccupied reaches in density 
of macroinvertebrates suggests that (over the observed range) macroinver- 
tebrate density alone did not determine dipper occupancy. Larger (wider) 
streams, however, typically have more substrate to support benthic inver- 
tebrates, so the total abundance of invertebrates should be greater in the 
larger streams that nesting dippers are more likely to occupy. Thus, the 
disparity between occupied and unoccupied streams in total abundance of 
macroinvertebrates is probably greater than the differences in invertebrate 
density we observed. 
The levels of annual survival of adult dippers we estimated include values 
slightly higher and lower than for other nonmigratory species of similar 
body size summarized by Martin and Li (1992) and Martin (1995), for the 
American Dipper as reported in other studies (39-56%; Ealey 1977, Price 
and Bock 1983), and for the White-throated Dipper (Marzolin 2002). The 
apparent annual survival we recorded is probably close to the actual value 
because we seldom missed resighting banded birds that were actually present 
in the study area (only one banded bird known to be alive after two years 
was not resighted in the intervening year; detection probability = 0.939). 
Our regular surveys of numerous streams should have revealed banded birds 
if they changed watersheds within our broad study area. We found several 
birds that had shifted territories within watersheds but none that changed 
watersheds (but see Osborn 2000), although dippers commonly move 
among watersheds in winter (Willson and Hocker 2008a). It remains pos- 
sible, however, that some birds moved to regions outside our study area. 
Limits to Abundance and Distribution 
Several factors may limit dipper abundance and distribution in an area. 
Nest sites are often thought to be the most important limiting factor, with 
some influence of prey availability (e.g., Kingery 1996). Evidence of nest-site 
limitation comes from rapid occupation of new sites such as bridges and nest 
boxes (Loegering and Anthony 2006, Osborn 1999) and lack of occupation 
after removal of nest sites (Backlund 1998). There is also evidence, how- 
ever, for the importance of prey. Campbell et al. (1997) stated that critical 
habitat is a food-rich stream, although nest sites help determine density 
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