Hooded Warbler
Ecoregional Scale Conservation Planning

Made possible through a partnership with the National Wetlands Research Center

Brown Thrasher (Toxostoma rufum)

The brown thrasher is a short-distance migrant found throughout eastern North America. Although populations in the CH and WGCP declined by 1.4 percent per year between 1966 and 2004 Table 005 (Table 005) , the species is not considered a Bird of Conservation Concern in either BCR Table 001 (Table 001) . The brown thrasher has a regional combined score of 13 and 15 in the WGCP and CH, respectively, and is considered a species warranting management attention in the latter BCR Table 001 (Table 001) .

Relative abundance of Brown Thrasher, derived from Breeding Bird Survey data, 1994 - 2003.
image courtesy of www.whatbird.com

Natural History:

The brown thrasher is a ground-foraging passerine associated with edge habitats throughout the eastern United States and Canada (Cavitt and Haas 2000). Breeding habitat includes a wide variety of vegetation types, but brown thrashers reach their highest densities in shrublands and mid-successional forests. Grand and Cushman (2003) found thrashers in Massachusetts predominately associated with the amount of scrub oak in the landscape. Rumble and Gobeille (2004) found no significant differences in brown thrasher occurrence among seral stages of cottonwood floodplains in South Dakota, though brown thrashers were detected most often in younger forest classes. Savanna restoration efforts increase the abundance of thrashers by reducing tree density (Davis and others 2000).

Nests are placed low in a tree or shrub and may even occur on the ground. Territory size and density vary depending on habitat quality (0.5-1.1 ha and 0.1-0.4/ha, respectively). The USFWS (Cade 1986) developed an HSI model for this species that included three site-specific factors: high woody stem density, low tree canopy cover, and high litter cover.

Model Description:

Our model for Brown Thrasher contains five factors:

  • landform
  • landcover
  • successional age class
  • edge occurrence
  • small (<2.5 cm d.b.h.) stem density
  • forest composition in a 10-km radius

The first suitability function combines landform, landcover, and successional age class into a single matrix (SI1) that defines unique combinations of these classes Table 040 (Table 040) . We directly assigned suitability index scores to these combinations based on habitat associations reported by Hamel (1992) for brown thrashers in the Southeast.

The brown thrasher is an edge species that inhabits thickets and hedgerows in deciduous forests. Because brown thrashers utilize both hard and soft edges, we defined edge (SI2) as the interface between pole age class forest and either herbaceous and non-forest landcovers (hard edge) or seedling and grass-forb age forest (soft edge). We required pole age forest sites to be adjacent to an edge to be considered suitable Table 041 (Table 041) . However, we relaxed this requirement for seedling-shrub and sapling stands, which we considered suitable irrespective of edge.

The brown thrasher occurs in habitats containing a high number of small stems (SI3). We fit a smoothed quadratic function Figure 020 (Figure 020) to HSI cutoff values from the USFWS HSI model for brown thrasher (Cade 1986; Table 042 Table 042 ) to quantify the relationship between small stem density and brown thrasher habitat suitability.

Although brown thrashers are associated with edges, they prefer modestly forested landscapes (Haas 1997). Therefore, we included forest composition (SI4) in our model to discount forest patches that were either isolated within a matrix of non-forest or occurred within heavily forested landscapes. We fit a Gaussian function Figure 021 (Figure 021) to landscape proportions reflecting this pattern and assumed 70 percent forested landscapes were associated with the maximum suitability index score (1.000; Table 043 Table 043 ).

We assumed brown thrashers used edge as a surrogate to early successional habitat, so we calculated suitability index scores separately for seedling-shrub-sapling and pole age class forests. In the former, the geometric mean of forest structure and landscape composition variables defines the suitability score. For the latter, we included edge occurrence as an additional factor in the calculation. We summed the age class-specific suitability index score to determine the overall suitability index score for all sites.

Seedling-shrub and sapling successional age classes: SIYoung: ((SI1 * SI3)0.500 * SI4)0.500

Pole successional age class: SIPole: ((SI1 * SI3)0.500 * SI4)0.500 * SI2

Overall SI = SIYoung + SIPole