AMERICAN DIPPERS NESTING NEAR JUNEAU, ALASKA 
along a stream. Price and Bock (1983) found territory size to be related 
to food density. Miller and Ralph (2005) found a higher density of nesting 
dippers on larger streams (third- and fourth-order) and a lower density on 
first-order streams. Stream size and susceptibility to drought affect stream 
use by the White-throated Dipper (Marzolin 2002). In Britain, studies of the 
White-throated Dipper on acidified streams with reduced abundance of prey 
support the importance of food availability. Osborn (1999) noted that good 
but unoccupied nest sites can be found on streams whose quality has been 
impaired by human activity. The relatively recent decline of dipper popula- 
tions in the Black Hills of South Dakota has been attributed to a combination 
of pollution, sedimentation, heavy grazing, dams, dewatering, and modern 
bridges without nest ledges (Backlund 1998, 2004). In addition, territorial 
aggression can limit the number of breeding pairs on a stream (Sullivan 
1973, Ealey 1977, Price and Bock 1983). 
In our study area, evidence that the availability of nest sites limits dipper 
populations includes the few territories centered on low-gradient streams 
where suitable nest sites have been provided by man-made structures, such 
as dams or bridges. These sites had sandy or silty substrates that dippers 
commonly avoid (e.g., Osborn 1999) because invertebrate prey is more 
abundant on coarse substrates (e.g., Willson and Hocker 2002a). Presum- 
ably, the dippers nesting in these sites needed to forage most intensively at 
some distance from the nest. During our study, dippers nested successfully 
in anthropogenic sites, and in general nest success has not been associated 
with particular habitat features (e.g., Loegering and Anthony 2006, Willson 
and Hocker 2008b). 
The probable importance of prey abundance to the dipper’s distribution in 
our area is indicated by the birds’ nesting consistently along larger streams, 
occasionally along intermediate-sized streams, and not along small streams, 
despite the existence of seemingly good nest sites. In 1977, however, after 
several years of unusually high snowfall (Juneau Forecast Office, http:// 
pajk.arh.noaa.gov), Robert H. Armstrong (pers. comm.) recorded very 
young dipper fledglings on a stream categorized here as low-flow. During 
our study, territorial aggression was observed to eliminate one breeding pair 
(Willson and Hocker 2008b). Moreover, territorial behavior clearly limited the 
size of the territory of another pair that foraged widely over two previously 
occupied but now vacant territories. These three factors taken together, an 
upper limit to dipper abundance and distribution in our area may be set by 
stream size (and inferred prey abundance) and nest sites, modified by ter- 
ritorial aggression. 
Low overwinter survival, however, especially when combined with poor 
nest success, reduced density below that upper bound. Apparent annual 
survival of marked adults varied from year to year, and low annual survival 
resulted in vacant territories and unoccupied streams. The existence of ter- 
ritories vacant in some years but occupied previously or subsequently, often 
with good nest success, suggests that sometimes there were too few birds 
to occupy all the territories. 
Even though dippers have dense insulating plumage and are able to func- 
tion well at very low temperatures (references in Kingery 1996, Willson and 
Hocker 2008a), cold winter weather appears to reduce their winter survival, 
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