The pileated woodpecker uses a wide variety of forest types across its range but is typically associated with the older successional age classes wherever it occurs (Bull and Jackson 1995, Annand and Thompson 1997). The key component to pileated woodpecker habitat is an abundance of large snags- the more the better. Different researchers define large differently (Renken and Wiggers 1989, Savignac and others 2000, Showalter and Whitmore 2002), but pileated woodpeckers are invariably associated with the largest size-class available. In Missouri, pileated woodpecker occurrence is associated with bottomland hardwood forest (Renken and Wiggers 1993); however, in east Texas, pileated woodpeckers are equally abundant in bottomland hardwoods, longleaf pine savanna, and mixed pine-hardwood stands, provided suitable snags were available (Shackelford and Conner 1997). Closed canopies (75–96 percent canopy cover) are the norm (Renken and Wiggers 1989). Pileated woodpeckers have large home ranges (53–160 ha); therefore, it is not surprising they are sensitive to forest area. Robbins and others (1989) observed no pileated woodpeckers in woodlots <42 ha and larger areas are likely required for breeding pairs. Schroeder (1982) considered 130 ha the minimum forest patch size for pileated woodpeckers.
The pileated woodpecker model contains six parameters:
- land cover
- successional age class
- large (>30 cm d.b.h.) snag density
- forest patch size
- percent forest in a 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 then used the pileated woodpecker habitat associations in Hamel (1992) to assign suitability index scores to these combinations.
Large (>30 cm d.b.h.) snags (SI2) are used for roosting, nesting, and foraging and are an important component of pileated woodpecker habitat. We fit a logistic function
to data from Renken and Wiggers (1993) on the relative density of pileated woodpeckers in sites with varying large snag densities to predict habitat suitability index scores based on this habitat feature
Last, we incorporated forest patch size (SI3) and percent forest in the local landscape (1-km radius; SI4) as predictors of pileated woodpecker habitat suitability. Large home ranges for this species necessitate large forest patch sizes. We fit a logarithmic function
to data from Robbins and others (1989) on the effect of forest patch size on occupancy rates by pileated woodpeckers
. We also included percent forest in the landscape (1-km radius; SI4) because small forest patches that may not be utilized in predominantly non-forested landscapes may provide habitat in predominantly forested landscapes due to their proximity to large forest blocks (Rosenberg and others 1999). Thus, we fit a logistic function
to data 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 maximum suitability index score from either SI3 or SI4 was used to account for the higher suitability of small forest patches in predominantly forested landscapes.
To calculate the overall suitability index score, we determined the geometric mean of SI scores for forest structure attributes (SI1 and SI2) and multiplied that by the maximum value of either forest patch size (SI3) or percent forest in the 1-km radius landscape (SI4) and calculated the geometric mean of that product.
Overall SI = ((SI1 * SI2)0.500 * Max(SI3, SI4))0.500