The white-eyed vireo is a small secretive songbird associated with dense vegetation in secondary deciduous scrub-shrub, wood margins, overgrown pastures, abandoned farmlands, streamside thickets, and even mid- to late-stage successional forests (Hopp and others 1995). White-eyed vireos share habitats with the blue-gray gnatcatcher, Carolina wren, gray catbird, and brown thrasher, but prefer later successional forest than yellow-breasted chat, prairie warbler, or Bell’s vireo.
White-eyed vireos in Texas breed in areas of shrubby vegetation (0-1 m) with dense foliage. Similarly, in Virginia they prefer habitats with an extensive undergrowth of shrubs, brambles, and saplings interspersed with taller trees (10-20 percent of area). Vireo densities are inversely related to vegetation height, foliage density at 12-15 m, density of pole trees, and percent canopy closure. Prather and Smith (2003) found them to be more abundant in tornado-damaged forest in Arkansas than in undamaged areas, and abundance was positively related to gap size in group-selection harvested bottomland forest in South Carolina (Moorman and Guynn 2001). Territory size (0.1–1.8 ha) and population density vary with habitat quality. Brood parasitism affects nearly half of all nests and may significantly reduce productivity. White-eyed vireos are more abundant in wide riparian strips of bottomland hardwood forest than narrow strips (Kilgo and others 1998).
The model for white-eyed vireo habitat suitability contains six parameters:
- successional age class
- edge occurrence
- canopy cover
- small (<2.5 cm d.b.h.) stem density
The first suitability function combined landform, landcover, and successional age class into a single matrix (SI1) that defines unique combinations of these classes
. We directly assigned habitat suitability scores to these combinations based on data from Hamel (1992) on the habitat associations of white-eyed vireos in the Southeast.
White-eyed vireos in older forest stands concentrate on edges (SI2) and other areas with dense vegetation. We used a 3 × 3 window to identify the interfaces between pole and sawtimber successional age class forest and herbaceous and non-forest landcovers (hard edge) or shrub-seedling, grass-forb, and sapling successional age class forest (soft edge). We assumed pole and sawtimber stands adjacent to these edges would have the highest suitability index score (1.000) but applied some residual suitability value (0.010) to areas not identified as edge habitats to compensate for small forest gaps and openings that may be utilized. Shrub-seedling and sapling stands were suitable habitat irrespective of edge
To further capture the association of white-eyed vireos with canopy gaps, we included modeled the effect of canopy cover (SI3) on suitability index scores as an inverse logistic function
that captured the absence of white-eyed vireos in close-canopied forests
Lastly, we fit a logistic function
to data from Annand and Thompson (1997;
on the influence of small (<2.5 cm d.b.h.) stem density (SI4) on relative density of white-eyed vireos to quantify the relationship between suitability index scores and this habitat feature.
Assuming this species uses edge as a surrogate to its preferred shrub-seedling and sapling habitats, we calculated suitability index scores separately for shrub-seedling-sapling and pole-sawtimber forest stands. In the former, the geometric mean of forest structure variables alone defines the suitability score. For the latter, landscape composition (namely, edge occurrence) was also a factor in the calculation.
Shrub-seedling and sapling successional age classes:
SIYoung: (SI1 * SI3 * SI4)0.333
Pole and sawtimber successional age classes:
SIOld: ((SI1 * SI3 * SI4)0.333 * SI2)0.500
To determine the overall suitability index score, we summed the age-class specific SIs:
Overall SI = SIYoung + SIOld