Hooded Warbler
Ecoregional Scale Conservation Planning

Made possible through a partnership with the National Wetlands Research Center

Worm-eating Warbler (Helmitheros vermivorus)
The worm-eating warbler breeds on the forested slopes of the eastern deciduous forest. It is notably absent from the Mississippi floodplain and the relatively flat forest-prairie ecotone immediately east of the Great Plains. Its preference for rugged terrain and its high-pitched insect-like song result in underestimations of its density from roadside surveys. Therefore, it is not surprising no credible trends are evident from BBS data for this species Table 005 (Table 005) . Nevertheless, the species is a Bird of Conservation Concern in both BCRs. However, PIF designates the worm-eating warbler as a management attention priority in the CH (regional combined score = 18) but only a planning and responsibility species in the WGCP (regional combined score = 15; Table 001 Table 001) .
Relative abundance of Worm-eating Warbler, derived from Breeding Bird Survey data, 1994 - 2003.
image courtesy of www.whatbird.com

Natural History:
The worm-eating warbler is a Neotropical migrant that breeds in forest interiors of the eastern United States (Hanners and Patton 1998). Minimum area requirements range from 21 ha in the mid-Atlantic (Robbins and others 1989) to >800 ha in Missouri (Wenny and others 1993). Worm-eating warblers nest on the ground along moderate to steep slopes (≥20º) containing dense (≥48 percent) shrub understories in mature deciduous and mixed deciduous-coniferous forests (Gale and others 1997). Both Artman and others (2001) and Blake (2005) found worm-eating warblers less abundant in recently burned stands due to the loss of leaf litter, a preferred nesting and foraging substrate. Canopy closure exceeded 95 percent in both Missouri (Wenny and others 1993) and Connecticut (Gale and others 1997).

Model Description:

The model for worm-eating warbler habitat suitability contains six parameters:

  • landform
  • landcover
  • successional age class
  • forest patch size
  • percent forest in the landscape, slope
  • small (<2.5 cm d.b.h.) stem 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 147 (Table 147) . We directly assigned habitat suitability scores to these combinations based on habitat associations reported in Hamel (1992)

We included slope (SI2) in our model because of the prevalence of steep slopes in the territories of worm-eating warblers. We defined slope classes based on data from Gale and others (1997) who identified the relative preference of various slopes for worm-eating warblers Table 148 (Table 148) .

We also included forest patch size (SI3) as a model parameter to account for the preference of worm-eating warblers for forest interiors. We fit a logistic function Figure 089 (Figure 089) to data from Robbins and others (1989) to quantify the relationship between forest patch size and habitat suitability Table 149 (Table 149) . The suitability of a forest patch is influenced not only by its size, though, but also its landscape context (SI4). In predominantly forested landscapes, small forest patch sizes not otherwise suitable may be used due to their proximity to a large forest block (Rosenberg and others 1999). We built a logistic function Figure 090 (Figure 090) based on the assumptions that landscapes with <30 percent forest were poor habitat (suitability index score ≤ 0.100) and landscapes with >70 percent forest were excellent habitat (suitability index score ≥ 0.900; Table 150 Table 150) . The maximum suitability index score of either SI3 or SI4 was assigned to each site to account for the higher suitability of small forest patches in heavily forested landscapes.

We relied on data from Wenny and others (1993) and Annand and Thompson (1997; Table 151 Table 151) to quantify the relationship between habitat suitability scores and small stem density (SI5; Figure 091 Figure 091) . We assumed worm-eating warblers occurred in forests with low stem densities, but these habitats had lower suitability scores than sites with well-developed understories characterized by dense stems.

To calculate the overall suitability index score, we determined the geometric mean of SI scores for forest structure (SI1 and SI5) and landscape composition (Max(SI2,SI3) and SI4) separately and then the geometric mean of these means together.

Overall SI = ((SI1 * SI5)0.500 * (Max(SI2, SI3) * SI4)0.500)0.500