Hooded Warbler
Ecoregional Scale Conservation Planning

Made possible through a partnership with the National Wetlands Research Center

Bell's Vireo (Vireo bellii)

The Bell’s vireo is a scrubland specialist that reaches the eastern limit of its range in the CH and WGCP. Throughout this region the species has been declining by 3.7 percent per year over the past 40 years, with declines most severe in the southern portion of the eastern range (-4.7, -6.6, and -10.1 percent annually in Missouri, Oklahoma, and the Ozark-Ouachita Plateau, respectively). The Bell’s vireo has a regional combined score of 15 in the CH and 16 in the WGCP, and PIF considers the species to require management attention in both regions Table 001 (Table 001) . The USFWS also recognizes Bell’s vireo as a Bird of Conservation Concern in both regions as well Table 001 (Table 001) .

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

Natural History:

Bell’s vireo is a small Neotropical migrant associated with dense, low, shrubby vegetation (Brown 1993). The bird uses a wide variety of early-successional scrubland habitats that meet these requirements (e.g., riparian woods, brushy fields, and regenerating forest). Most research on this species comes from the Far West, where the species is alternately described as a riparian specialist (particularly the federally-endangered subpopulation of least Bell’s vireos in California) or scrub-shrub generalist. Bell’s vireos nest in dense shrub or understory vegetation 0.5-1.5 m above the ground, making their nests susceptible to both terrestrial and avian predators. Predation and brood parasitism are the primary causes of nest failure (Budnik and others 2000, Budnik and others 2002, Powell and Steidl 2000). Territory size varies with habitat quality and ranges from 0.1 to 1.6 ha. Increasing the density of large shrub patches has been proposed as an effective management practice for improving Bell’s vireo habitat in Missouri (Budnik and others 2002).

Model Description:

The model for Bell’s vireos contains six factors:

  • landform
  • landcover
  • successional age class
  • interspersion of forest and open areas
  • edge
  • 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 021 (Table 021) . We directly assigned suitability index values to these combinations based on data from Hamel (1992) relating vegetation types and successional age class to habitat suitability estimates for Bell’s vireos.

Both landcover and age class data were used to identify upland shrublands in grassland landscapes, the preferred habitat for this species in its eastern range (Budnik and others 2000). We used a 10-ha moving window (an average home range size; Budnik and others 2000) to assess the interspersion of shrubland and grassland habitats (SI2). We assumed an area containing 50 percent of each habitat type was ideal (i.e., suitability index score = 1.000; Table 022 Table 022) . We also assumed suitability declined symmetrically as landscape proportions moved away from this optimal configuration. Landscapes lacking either shrublands or grasslands were considered unsuitable and assigned a suitability index score of 0.000.

Bell’s vireos use a variety of young woody habitats (Brown 1993); however, birds will nest along the edges of sapling stands and in hedgerows as well (Budnik and others 2002). Therefore, we included edge (SI3) as a parameter in the Bell’s vireo habitat suitability model. To identify edges, we examined the eight pixels surrounding each sapling age-class pixel to determine if any were classified as shrub-seedling or grass-forb age class or as herbaceous or non-forest landcover class. If so, the central pixel in the 3 × 3 pixel window was assigned a suitability index score of 1.000. If not, it was assigned a 0.000. We assigned grass-forb and shrub-seedling pixels a suitability index score of 1.000 irrespective of edge Table 023 (Table 023) . Similarly, we always assigned pole and sawtimber pixels a suitability index score of 0.000 irrespective of edge.

We also included small stem density (SI4) as a component of the overall Bell’s vireo habitat suitability model because of the importance of dense woody shrub cover for this species. Farley (1987) measured an average of 9.8 stems >2 mm per 1 m diameter plots (~392000 stems/ha) in Bell’s vireo territories. This relatively high stem value included woody and non-woody stems of all sizes greater than 2 mm; therefore, we assumed an eighth of these stems were woody and <2.5 cm d.b.h. (49000) and that this number represented optimal habitat (suitability index score = 1.000; Table 024 Table 024 , Figure 010 Figure 010) .

To calculate the overall suitability index score for Bell’s vireo, we first determined the geometric mean of the suitability indices related to forest structure (SI1 and SI4) and landscape attributes (SI2 and SI3) separately and then determined the geometric mean of these values together. Because SI3 only applies to sapling habitats, suitability index scores were calculated separately for sapling stands than grass-forb and shrub-seedling stands and these scores were added together to determine the final suitability index score for the entire landscape.

For grass-forb and shrub-seedling habitats:
SIGF and SS= (((SI1 * SI4)0.500) * (SI2))0.500

For sapling habitats:
SISap = ((SI1 * SI4)0.500 * (SI2 * SI3))0.500

SI = SIGF and SS + SISap