From beneath the tangles and fallen trees, darting back-and-forth in the moist and mossy understory, the Pacific Wren sings loudly from its favourite perches and “seems as much a part of the forest floor as the mosses, huckleberry vines, huge logs, and upturned roots of his surroundings” ( Taylor and Shaw 1927 ). A superb songster—the “pinnacle of song complexity” ( Kroodsma 1980 )—it is more often heard than seen. It is a small, brown, cryptically-colored wren that generally inhabits dark, moist coniferous forests in western North America, although it can be found breeding on cliff faces on treeless islands of Alaska and the Aleutians.
Until recently the Pacific Wren, the Winter Wren (Troglodytes hiemalis) in eastern North America, and the Eurasian Wren (or “the Wren” in Europe; T. troglodytes) were together recognized as a single species. Patterns of genetics ( Drovetski et al. 2004 ) and vocalizations ( Kroodsma 1980 , Kroodsma and Momose 1991 , Kroodsma and Brewer 2005 , Toews and Irwin 2008 ) demonstrate, however, that the similarities in breeding plumage and morphometric traits conceal deep divides within this group. In particular, research from an area of range overlap between T. pacificus and T. hiemalis in western Canada indicates that the two are genetically and phenotypically distinct where they co-occur and that the strong differences noted in their song, which are maintained in this area of range contact, provide a potentially important reproductive barrier between the two ( Toews and Irwin 2008 ).
The Pacific Wren breeds in temperate forests west of the eastern foothills of the Rocky Mountains, ranging from California and Utah to the southern coast of Alaska and the Aleutian Islands. There are seven currently recognized subspecies, two of which occur throughout its mainland distribution, with the rest of the subspecies inhabiting islands off the Alaskan coastline.
The species is unique among North American wrens in its association with old-growth forests. It uses old-growth structures (snags, root masses, downed trees, and the bases of large standing trees) for nesting, foraging, and roosting. Breeding territories are primarily found in forests along rivers and streams and, at least in the rainforests along the Pacific Coast, the presence of Pacific Wrens is correlated with riparian areas enriched with salmon-derived nutrients ( Field and Reynolds 2011 ). Clear-cutting and some types of partial logging reduce habitat suitability for the Pacific Wren, which tends to avoid forest edges, and it is one of a number of species that is likely harmed by forest fragmentation in western North America ( Brand and George 2001 ). Based on habitat change from pre-settlement times, Pacific Wren numbers are estimated to have declined in some regions Raphael et al. 1988 ) and many recent trends (from 1999–2009) indicate some populations in Washington and British Columbia are decreasing ( Sauer et al. 2011b ). Most monitoring methods are not particularly well suited for tracking Pacific Wren numbers and questions remain about how habitat alteration and future climate change will affect this species ( McRae et al. 2008 ). Further research is recommended to address these questions as well as a number of other knowledge gaps.
Thorough studies have been conducted on the Pacific Wren in North America, including: Rice et al. 1999b , Drovetski et al. 2004 , Gomez et al. 2005, and Toews and Irwin 2008 on systematics; Kroodsma 1980 , Van Horne 1995 , and Toews and Irwin 2008 on song; Mclachlin 1983 , Van Horne and Bader 1990 , and Field and Reynolds 2011 on food habits; Heath 1920 , Bent 1948b , Mclachlin 1983 , Waterhouse 1998 , Willson and Gende 2000 , Waterhouse et al. 2002a , De Santo et al. 2003 and Evans Ogden et al. (in press) on breeding biology, behavior, and habitat use; and numerous community studies examining the effects of timber harvesting on Pacific Wrens, especially Rosenberg and Raphael 1986 , Lehmkuhl et al. 1991 , Hejl and Paige 1994 , Mcgarigal and McComb 1995 , Hutto and Young 1999 , Brand and George 2001 , and McRae et al. 2008 .