Hooded Warbler
Ecoregional Scale Conservation Planning

Made possible through a partnership with the National Wetlands Research Center

Brown-headed Nuthatch (Sitta pusilla)

The brown-headed nuthatch is a resident species of mature pine forests along the Piedmont and Coastal Plains of the southeastern United States. The species is a USFWS Bird of Conservation Concern in the WGCP Table 001 (Table 001) , where it has a regional combined score of 19. The brown-headed nuthatch is a rare breeder in the CH (regional combined score = 19), and PIF deems this species warrants critical recovery efforts in this region.

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

Natural History:

The brown-headed nuthatch is closely associated with pine: it breeds in mature pine forests and forages almost exclusively in pine trees (>98 percent of observations; Withgott and Smith 1998). Although often associated specifically with the longleaf pine savanna characteristic of red-cockaded woodpecker and Bachman’s sparrow habitat, the brown-headed nuthatch has a broader niche than these species (Hamel 1992, Dornak and others 2004). Brown-headed nuthatch habitat is defined by two habitat elements: mature pines for foraging and cavities for nesting (Wilson and Watts 1999, Dornak and others 2004). Specific pine species composition is not as critical as d.b.h., with an average d.b.h. of 25.6 cm optimal (O’Halloran and Conner 1987 cited in Dornak and others 2004). Brown-headed nuthatches primarily nest in large d.b.h. snags <3 m tall and may require 7–8 snags/ha to ensure adequate nest and roost sites, particularly in the presence of interspecific competition for cavities. In urban areas, brown-headed nuthatches readily adopt nest boxes and may use other man-made cavities (e.g., streetlights).

Brown-headed nuthatches prefer open pine stands with few hardwoods (≤17.4 stems/ha and basal area ≤5 m2/ha) and an open midstory (Wilson and Watts 1999). Optimal canopy closure is highly variable (15-85 percent), but stands with closed canopies are not preferred (O’Halloran and Conner 1987, Wilson and Watts 1999). Undergrowth is typically sparse (~35 percent; Dornak and others 2004). Nuthatches regularly breed at low densities in suboptimal habitats, including stands with small pines, a large fraction of hardwoods, and dense understories (Withgott and Smith 1998). Area sensitivity does not appear to be an issue for this species, which is not an acceptable host for the brown-headed cowbird (Withgott and Smith 1998).

Model Description:

The brown-headed nuthatch model contains six factors:

  • landform
  • landcover
  • successional age class
  • snag density
  • small (<2.5 cm d.b.h.) stem density
  • hardwood basal area

The first suitability function combines landform, landcover, and successional age class into a single matrix (SI1) to define unique combinations of these classes Table 044 (Table 044) . We directly assigned suitability index scores to these combinations based on brown-headed nuthatch habitat associations described by Hamel (1992).

We included snag density (SI2) in our habitat suitability index model because of the importance of cavities to this species. We assumed suitability index scores for brown-headed nuthatch habitat was 0.000 when ≤8 snags of any size were present (Dornak and others 2004). We fit a logistic function Figure 022 (Figure 022) to data from Wilson and Watts (1999; Table 045 Table 045 ) to quantify the relationship between snag density and suitability index scores.

We also used small stem density as a function (SI3) in the habitat suitability model to account for the preference of brown-headed nuthatches for open understories. We fit an inverse logistic function Figure 023 (Figure 023) to hypothetical data reflecting this preference Table 046 (Table 046) .

Lastly, we incorporated hardwood basal area (SI4) as a model factor, as birds are less abundant in habitats with a greater hardwood component. Again, we relied on data from Wilson and Watts (1999; Table 047 Table 047 ) to develop an inverse logistic function to describe the relationship between hardwood basal area and habitat suitability index score Figure 024 (Figure 024) .

To determine the suitability index score for brown-headed nuthatch habitat, we calculated the geometric mean of the four individual functions related to forest structure attributes.

Overall SI = (SI1 * SI2 * SI3 * SI4)0.250