The Wilson Journal of Ornithology 124 ( 2 ): 270 - 276 . 2012 
MATING AND BREEDING SUCCESS DECLINE WITH ELEVATION 
FOR THE PACIFIC WREN (TROGLODYTES PACIF1CUS) IN COASTAL 
MOUNTAIN FORESTS 
LESLEY J. EVANS OGDEN. 1 MICH A EL A MARTIN. 12 AND KATHY MARTIN 134 
ABS I RAC T. We studied the population ecology of Pacific Wrens t Troglodytes pacificus) in 2003 and 2004 breeding 
across elevations from 1(H) to 1,300 in in coastal mountain forests in southwestern British Columbia. Canada to examine if 
this spec.es «s adapted to upper montane and subalpine habitats. We found fewer territories at high elevation, a higher 
proporimn ot unmated males, fewer nests per mated male, and no returns of banded adults or juveniles. The breeding season 
was 61 ■*> shorter (31 vs. 79 days), and mass of nestlings (at 11-12 days of age) and nest survival were lower at hiah 
elevation compared to lower elevation sites. Clutch size, incubation and nestling periods, parental provisioning rates of 
nestlings, and adult morphology did not vary with elevation. Annual fecundity measures declined with increasing elevation 
with no apparent compensatory increases in other vital rates such as survival of adults or offspring. Received 27 October 
2011. Accepted 23 December 2011. 
I lie Pacific Wren (Troglodytes pacificus), a 
small migratory songbird that breeds in coastal 
coniferous forests of western North America, was 
recently described as a species distinct from (he 
Winter Wren (T. hiemalis) in the rest of North 
America and the Eurasian Wren (T. troglodytes) 
in Europe (Wesolowski 1983, Toews and Irwin 
2008, Chesser et al. 2010). Pacific Wrens are 
common birds of northern temperate coniferous 
and mixed forests of North America, but there are 
few studies of their breeding ecology and nesting 
success (McLachlin 1983, Van Home and Bader 
1990. Waterhouse 1998, De Santo et al. 2003). 
The Pacific Wren breeds across an elevation 
gradient from sea level to upper montane and 
subalpine habitats along the west coast of North 
America (Toews and Irwin 2008). 
High mountain areas are characterized by 
variable weather, cold temperatures, and short 
breeding seasons (Martin 2001). At least 90 bird 
species breed across wide elevation gradients, 
some of which range from sea level to alpine 
habitats, but their ecology, behavior, and life 
history at upper elevation limits arc poorly 
understood. Short breeding seasons at high 
elevations translated into lower annual fecundity 
for the few species studied, particularly when the 
'Centre lor Applied Conservation Research. Department 
ot Fores. Sciences. 2424 Main Mall. University of British 
Columbia. Vancouver. BC VfiT IZ4. Canada. 
-Centre for Wildlife Ecology. Department of Biological 
ciences, Simon Fraser University. Burnaby. BC V5A IS6 
Canada. 
V4K E 7N^c^ula anatl:, ' ^ RohcrtSOn Road ’ Delta. BC 
J Corresponding author; e-mail: Kathy.Martin@ubc.ca 
270 
ability to initiate nesting was influenced by snow¬ 
melt phenology (Martin and Wiebe 2004. Martin 
et al. 2009, Wilson and Martin 2010). Birds 
breeding al high elevations appear to compensate 
lor reduced annual fecundity by increasing their 
per capita parental care, thus improving juvenile 
survival (Badyaev and Ghalarnor 2001). Body 
size ot adults and egg size both increase with 
increasing elevation (Bears et al. 2008. Zeng and 
Lit 2009, Lu et al. 2010). Several songbird species 
trade reduced annual fecundity for increased 
survival of both adults and young in high 
elevation populations (Bears et al. 2009. Martin 
et al. 2009. Camllcld et al. 2010). These studies 
louml intra-specific differences with elevation as 
high elevation populations shifted to a “slower 
lile history with a longer life span, fewer offspring 
per breeding season, but greater investment in 
parental care. 7 he length of the breeding season 
sharply decreases with increasing elevation due to 
later snow melt and shorter vegetative growth 
seasons, and birds increase their per capita 
parental investment. Birds at higher elevations 
are larger, heavier, and have higher survival 
(Bears et al. 2009. Martin et al. 2009, Cornfield 
et al. 2010). 
We investigated variation in the breeding 
ecology of the Pacific Wren along an altitudinal 
gradient from 100 to 1.300 m. We predicted that 
as elevation increased: (1) breeding season 
duration, the number of broods, and/or clutch 
size would decrease; (2) duration of incubation 
and nestling periods would increase: (3) per capita 
parental care ot nestlings and offspring mass at 
fledging would increase; (4) birds would have 
higher survival; and (5) mating status would not 
