The yellow-throated warbler breeds in two distinct habitat types: mature bottomland hardwoods forest and dry, upland oak-pine forest (Hall 1996). The species is more common in the former. Birds exhibit a strong affinity for cypress along the Coastal Plains, but along inland rivers prefer sycamores (Hall 1996, Gabbe and others 2002). Where Spanish moss is found, it is used for both foraging and nesting (Hall 1996). Elsewhere, the warbler forages by creeping along limbs and probing leaf clusters and pinecones. The yellow-throated warbler as both an interior and edge species and may occur in woodlots as small as 6 ha (Blake and Karr 1987). Robbins and others (1989) associated this species with large (>38 cm d.b.h.) tree density, forest in a 2-km buffer, and coniferous canopy cover.
Our model of yellow-throated warbler habitat suitability contains six factors:
- successional age class
- large (>50 cm d.b.h.) tree density
- distance to water
- percent forest in the landscape (1-km radius)
The first suitability function combines landform, landcover, and successional age class into a single matrix (SI1) that defines unique combinations of these classes
. We directly assigned suitability index scores to these combinations based on habitat association outlined in Hamel (1992) for yellow-throated warblers in the Southeast.
We also incorporated large tree density (SI2) into our model of yellow-throated warbler habitat suitability because of the affinity of this species for nesting and foraging in large trees (Hamel 1992, Robbins and others 1989). Lacking data points from the literature to fit a curve, we assumed habitat suitability scores were logistically related to large tree density up to 50 trees/ha and remained optimal above this threshold (suitability index score = 1.000;
Yellow-throated warblers nest near water (Hall 1996, Hamel 1992). Thus, we included distance to water (SI3) in the habitat suitability model. We assumed sites closer to water had a higher suitability. Lacking quantitative data on the potential effect of water on habitat suitability, we assumed the yellow-throated warbler had a territory size similar to the Acadian flycatcher but not as dependent on water as this species. Therefore, we assumed all sites <120 m from water to be optimal (suitability index score = 1.000), and we allowed habitat suitability index scores to decline more slowly for the yellow-throated warbler than the Acadian flycatcher
Yellow-throated warblers respond to the percent of forest in the landscape (SI4). Thus we fit a logistic function
to a dataset 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;
To calculate the overall suitability index score, we determined the geometric mean of suitability index scores for forest structure (SI1 and SI2) and landscape composition attributes (SI3 and SI4) separately and then the geometric mean of these means together.
Overall SI = ((SI1 * SI2)0.500 * (SI3 * SI4)0.500)0.500