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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 124. No. 2. June 2012 
TABLE 1. Number of territories, mating status, nesting success, and survival of Pacific Wrens in relation to elevation 
on Mount Seymour, British Columbia, Canada. 
Low elevation (100-390 m) 
High elevation (750-1.270 m) 
2003 
2004 
2003 
2004 
A. Territories and mating status 
Number of territories 
21 
18 
10 
11 
Number of unmated territorial males 
0 
0 
3 
9 
Number of banded males 
15 
15 
7 
5 
Average total nests/malc 
2.4 
4.0 
1.3 
2.6 
Percent successful nests/male 
25 
24 
0 
0 
B. Annual local survival 
Adults 
Number banded 
28 
19 
Number returning the following year 
9 
0 
Juveniles 
Number banded 
50 
6 
Number returning the following year 
1 
0 
We estimated the date-of-firsi-egg-laid by 
backdating (or nesls discovered after onset of 
incubation and, where hatching and/or Hedging 
events were observed, using average periods of 15 
and 17 days for incubation and nestling periods, 
respectively (Hejl et al. 2002). 
Data Analysis .—We used R Version 2.4.0 (R 
Development Core Team 2006) for all statistical 
analyses. Clutch initiation dates were standardized 
between years by subtracting the yearly median. 
Clutch sizes were not normally distributed and we 
used a generalized linear model with a Poisson 
distribution to analyze the response of clutch size 
to elevation, year, and clutch initiation date. 
Linear models were used to evaluate the response 
of clutch initiation (date-of-first-egg-laid). provi¬ 
sioning rates, and incubation and nestling periods 
to elevation. Log and square-root transformations 
improved model fit for clutch initiation and 
nestling period models, respectively. We calcu¬ 
lated daily nest survival rates using the logistic 
exposure method (Shaffer 2004). ANOVA was 
used to test tor differences in morphology and 
condition (wing chord, tarsus, body mass) of 
adults and pre-fledging nestling mass among three 
elevation categories (high, middle, and low 
elevations) and sex (adults only). Alpha was set 
at 0.05 and means are reported ± SE. 
RESULTS 
We located 22 and 24 active nests (containing 
eggs or nestlings), 73 and 72 potential nests 
TO"* COns,ructed and/or maintained and 
attended by a territorial male) and one and three 
recently failed nests during the 2003 and 2004 
breeding seasons, respectively. Active nests were 
distributed over an elevation range from 102 to 
1.088 m. We did not find active nests above 
UOO m, but did locate one potential nest at 
1.270 m and territorial males above 1.200 m in 
both years. We captured the majority of adults and 
nestlings at active nests in 2003 and 2004. We 
found fewer territories in both years, and fewer 
males successfully obtained one or more mates at 
higher elevation sites (Table 1). Our sample of 
banded males at low elevation had more nests 
(including both active and potential nests) and a 
higher proportion of their nests were successful 
compared to high elevation (Table 1). 
Clutch Initiation and Clutch Size .—Clutches 
were initiated over a span of 79 days (40 nests) at 
low elevation, and over 31 days (6 nests) at high 
elevation. There was a tendency for later clutch 
initiation at high elevations, but clutch initiation 
dates did not differ between elevations {F/ j: r 
3.26. P = 0.08). Nests at low elevations had wo 
peak clutch initiation periods, whereas nests at 
high elevation had only one peak at a similar time 
as the second peak period at low elevation 
(Fig. I A). 
Mean clutch size (including first and re-nest 
attempts and second broods) was 5.5 ± 0.10 (n - 
40) and did not vary with elevation (Dev Resit !,. 
= 0.15, P = 0.70), clutch initiation dates (Dev 
Residue = 0.411, P = 0.52), or with year (Dev 
Residue = 0.01. P = 0.93: Fig IB). Fledglings 
produced per eggs laid declined from an average 
of 0.5 at 200-400 m to 0 above 600 m. It was not 
