Hooded Warbler
Ecoregional Scale Conservation Planning

Made possible through a partnership with the National Wetlands Research Center

Bewick's Wren (Thryomanes bewickii)

Bewick’s wren was once a common resident throughout the Southeast and Mid-Atlantic. However, its range has steadily contracted over the last century, and today the species is virtually absent east of the Mississippi River (Kennedy and White 1997). Breeding Bird Survey data from the USFWS Region 4 indicates populations have declined 12.8 percent per year over the last 40 years (Sauer and others 2005). The decline of this species coincided with the range expansion of the house wren, which often destroys Bewick’s wren nests and has been implicated in its decline (Kennedy and White 1996). Bewick’s wren is a Bird of Conservation Concern. PIF identifies the species as needing critical recovery in the WGCP (regional combined score = 16) and immediate management attention in the CH (regional combined score = 15) Table 001 (Table 001) .

Relative abundance of Bewick's Wren, derived from Breeding Bird Survey data, 1994 - 2003.
image courtesy of www.whatbird.com

Natural History:

The Bewick’s wren is a small resident passerine that breeds in a variety of vegetation types, including brushy areas, scrub and thickets in open country, and open and riparian woodlands (Kennedy and White 1997). This plasticity has produced conflicting reports of habitat associations in the literature (e.g., dry vs. riparian, open woodlands vs. shrub thickets); however, the species likely responds most strongly to the availability of nest sites. Bewick’s wrens nest in cavities or opportunistically in crevices up to 10 m high. In the eastern portion of its range, Bewick’s wrens often occur near human habitations, particularly farmlands. Population declines of this species may be, in part, a result of competition with house wrens, which destroy Bewick’s wren nests in areas where the two species’ ranges overlap (Kennedy and White 1996). Bewick’s wrens mainly occur in grassland scrub while house wrens occur in secondary growth on abandoned agricultural land and residential areas. Both species are able to exploit the full range of these habitat types, though, and populations of both expanded as these latter habitat types increased. However, as scrub habitats declined, Bewick’s wrens may have declined because its main source habitat was no longer abundant.

Model Description:

Our model for Bewick’s wren contains five factors:

  • landform
  • landcover
  • successional age class
  • interspersion of forest and open habitatsnag density.

The first suitability function combines landform, landcover, and successional age class into a single matrix (SI1) that defines unique combinations of these classes Table 025 (Table 025) . We then directly assigned habitat suitability index score to these combinations based on data from Hamel (1992) on the relative quality of Bewick’s wren habitat based on vegetation type and successional age class.

We also considered interspersion of forest and grassland habitats (SI2) to be important for this species, as Bewick’s wrens are most abundant in semi-open areas containing ~60 percent woodland (Pogue and Schnell 1994; Table 026 Table 026) . We used data from Pogue and Schnell (1994) to define suitability index values along the diagonal axis of our interspersion table (i.e., where forest and grassland totaled 100 percent) and completed the rest of the table based on these values.

We also included snag density (SI3) in our model of Bewick’s wren habitat because as a secondary cavity nester, the species is believed to respond strongly to nest site availability. We assumed higher snag densities would decrease competition with other cavity nesters, improving habitat quality. Specific data relating snag density to Bewick’s wren habitat suitability was not available. Therefore, we adopted data on average snag density (16.4 snags/ha) within home ranges of house wrens (Sedgwick and Knopf 1990, Rumble and Gobeille 2004; Table 027 Table 027 ), a similarly-sized secondary cavity nester, for use in building a logistic function to quantify this relationship Figure 011 (Figure 011) .

To calculate the overall suitability index score, we first calculated the geometric mean of the two suitability indices related to forest structure attributes (SI1 and SI3), and then the geometric mean of this result and the SI related to interspersion (SI2).

SI = ((SI1 * SI3)0.500 * SI2)0.500