The yellow-throated vireo breeds along the edges of mature forest stands, and their abundance may even decline within forest interiors (Rodewald and James 1996). Appropriate edges include streams, rivers, swamps, and roads. Parks, orchards, and suburban habitats may also be used (Rodewald and James 1996). Yellow-throated vireos use both bottomland and upland sites but are restricted to deciduous and mixed-forest habitats. As a forest edge species, they are not area sensitive and may even benefit from canopy gaps. However, Robbins and others (1989) observed a positive relationship between yellow-throated vireo abundance and forest cover within a 2 km buffer. Similarly, yellow-throated vireos did not use riparian forests strips <70 m wide in east Texas (Conner and others 2004). Thus, yellow-throated vireos prefer canopy gaps within forested landscapes. The key component of yellow-throated vireo habitat is canopy structure, and birds select taller trees (>20 m) than other vireos (James 1976). Robbins and others (1989) also noted a positive relationship between yellow-throated vireo abundance and canopy height. Specific tree species do not affect selection (Gabbe and others 2002).
Our yellow-throated vireo habitat suitability model contains six factors:
- successional age class
- forest patch size
- percent forest in the landscape (1-km radius)
- canopy cover
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 habitat suitability scores to these combinations based on the relative rankings of yellow-throated vireo habitat associations described in Hamel (1992).
Although a forest edge species, yellow-throated vireos are affected by forest area (SI2) and the percent forest in the landscape (SI3). We fit a logistic function
to data from Blake and Karr (1987) and Kilgo and others (1998) to describe the relationship between forest patch size and yellow-throated vireo habitat suitability
. Similarly, we used a logistic function to predict habitat suitability from the percent forest cover in a 1-km radius landscape
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;
The affinity of yellow-throated vireos for canopy gaps led us to incorporate canopy cover in the habitat suitability model for this species (SI4). We fit a smoothed quadratic function
to data from Kahl and others (1985;
on the relative density of yellow-throated vireos at varying canopy closures and assumed Kahl’s optimal designation of canopy cover (80–90 percent) was associated with maximal suitability index scores (1.000). Further, we assumed yellow-throated vireo habitat suitability was reduced as canopy cover moved away from these optima.
To calculate the overall suitability index score, we determined the geometric mean of suitability index scores for forest structure (SI1 and SI4) and landscape composition attributes (SI2 and SI3) separately and then the geometric mean of these means together.
Overall SI = ((SI1 * SI4)0.500 * (SI2 * SI3)0.500)0.500