We studied the population ecology of Pacific Wrens (Troglodytes pacificus) in 2003 and 2004 breeding across elevations from 100 to 1,300 m in coastal mountain forests in southwestern British Columbia, Canada to examine if this species is adapted to upper montane and subalpine habitats. We found fewer territories at high elevation, a higher proportion of 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 high 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.