Hooded Warbler
Ecoregional Scale Conservation Planning

Made possible through a partnership with the National Wetlands Research Center

Blue-gray Gnatcatcher (Polioptila caerulea)

The blue-gray gnatcatcher is a short-distance migrant that occurs throughout eastern North America and the American Southwest. Blue-gray gnatcatcher populations are relatively stable in both the CH and WGCP (table 5), and the USFWS does not recognize this species as a Bird of Conservation Concern in either region Table 001 (Table 001) . The species requires management attention in the CH (regional combined score = 14) but receives no special designation in the WGCP (regional combined score = 13; Table 001 Table 001) .

Relative abundance of Blue-gray Gnatcatcher, derived from Breeding Bird Survey data, 1994 - 2003.
image courtesy of www.whatbird.com

Natural History:

The blue-gray gnatcatcher is a small passerine that inhabits a wide range of woodland types, ranging from shrubland to mature forest (Ellison 1992). It prefers deciduous habitats and is rare or absent from evergreen forests. The species attains its highest numbers in mesic and low-lying areas, but does occur in xeric forests and along ridges.

Kershner and others (2001) did not identify any specific microhabitat requirements for this species in Illinois, and considerable variation in nest height (0.8-24.4 m) and territory size (0.5-8 ha) has been documented across the range.

Although often associated with edges, the species may be area-sensitive (Knutson 1995, Kilgo and others 1998). Nest success was greater for nests placed higher and farther from an edge in Illinois (Kershner and others 2001) but did not differ between bottomland hardwood stands and cottonwood plantations in the Mississippi Alluvial Valley (Twedt and others 2001). Blue-gray gnatcatcher abundance was higher in bottomland hardwood stands surrounded by fields than those surrounded by pine (Kilgo and others 1998).

Model Description:

The habitat suitability index model for blue-gray gnatcatchers contains seven variables in five functions:

  • landform
  • landcover
  • successional age class
  • forest patch size
  • percent forest in a 1-km radius landscape
  • edge
  • basal area

The first suitability function combines landform, landcover, and successional age class into a single matrix (SI1) that defines unique combinations of these classes Table 032 (Table 032) . We directly assigned suitability index scores to these combinations based on data from Hamel (1992) on the relative quality of vegetation associations and successional age classes for blue-gray gnatcatchers. We adjusted Hamel’s values for shrub-seedling and sapling-aged stands to account for the higher densities observed in young forests by Thompson and others (1992) and Annand and Thompson (1997).

We included forest patch size (SI2) as a model parameter to account for the area-sensitivity of blue-gray gnatcatchers. We fit a logarithmic function Figure 015 (Figure 015) to data from Robbins and others (1989) on the probability of occurrence for blue-gray gnatcatchers in stands of various sizes Table 033 (Table 033) . Nevertheless, the actual use of a forest patch reflects both its area and its landscape context (SI3). In predominantly forested landscapes, a small forest patch that may not be otherwise suitable may be occupied due to its proximity to a larger forest block (Rosenberg and others 1999). Because gnatcatchers are also associated with edges, though, they may not be as abundant in predominantly forested landscapes that lack significant edge habitat. Thus, we fit a Gaussian function Figure 016 (Figure 016) to hypothetical data that captured this relationship Table 034 (Table 034) . We assumed large (10-km radius) landscapes with 70–80 percent forest were optimal (suitability index score = 1.000) and suitability declined as the proportion of forest in the landscape moved away from this ideal. The maximum suitability score of either SI2 or SI3 was used to simultaneously account for patch area and landscape composition.

We also included edge (SI4) in our habitat suitability model because of the association of blue-gray gnatcatchers with edges within large forest blocks. Blue-gray gnatcatchers nest along both hard edges and soft edges (typically within 30 m; Kershner and others 2001); therefore, we defined edge as the interface between sapling, pole and sawtimber stands and either herbaceous and non-forest landcovers or seedling and grass-forb age class stands. We utilized a 7 × 7 moving window to identify where these adjacencies occurred but recognized blue-gray gnatcatchers are not restricted to edge habitats and applied a residual suitability index score (0.010) to sites that did not meet this criterion Table 035 (Table 035) .

We fit a quadratic function to data from Annand and Thompson (1997) on the response of blue-gray gnatcatchers to basal area (SI5; Table 036 Table 036 , Figure 017 Figure 017 ), reflecting the preference of blue-gray gnatcatchers for open forest conditions.

To calculate the suitability index score for sapling, pole, and sawtimber age classes, we determined the geometric mean of SI scores for forest structure (SI1 and SI5) and landscape composition attributes (Max(SI2, SI3) and SI4) separately and then the geometric mean of these means together. Because edge occurrence (SI4) was not applicable to the shrub-seedling age class, we calculated suitability index scores separately for this age class and summed across age classes to determine the overall suitability index score for the landscape.

Sapling, pole, and sawtimber successional age classes: SIOld = (((SI1 * SI5)0.500) * ((Max (SI2 or SI3)) * SI4)0.500)0.500

Shrub-seedling successional age classes: SIShrub = ((SI1 * SI5)0.500 * (Max (SI2 or SI3)))0.500

Overall SI = SIOld + SIShrub