The red-headed woodpecker is one of the most recognizable birds of the eastern United States and southern Canada, but few in-depth studies of the species have been conducted (Smith and others 2000). Nesting habitat consists of deciduous woodlands, including upland and bottomland hardwoods, riparian strips, open woods, open wooded swamps, groves of dead and dying trees, orchards, shelterbelts, parks, open agricultural lands, savannas, forest edge, roadsides, and utility poles (Smith and others 2000). The species prefers xeric sites containing large, tall trees, high basal area, and a sparse understory.
Red-headed woodpeckers exhibit seasonal shifts in habitat use. Red-headed woodpecker population dynamics are linked to annual fluctuations in oak acorn crops, and migration occurs in northern and western populations when hard mast is limited (Rodewald 2003). More locally, winter territories are established around small food caches within forest interiors; breeding territories are larger (3.1–8.5 ha in Florida) and concentrated along edges (Smith and others 2000).
Red-headed woodpecker occurrence varies with mean patch dimension, edge density of agricultural land, and the area of urban landcover (Lukomski 2003). The red-headed woodpecker is a primary cavity excavator, and snag availability may drive habitat selection (Giese and Cuthbert 2003). The species is often associated with high snag densities (Conner and others 1994) in mature stands near openings (Conner and Adkisson 1977, Brawn and others 1984). Snag density and dead elm basal area distinguish nest sites from random sites in Minnesota (Giese and Cuthbert 2003). Similarly, loblolly pine stands with standing and down dead woody debris removed contain fewer red-headed woodpeckers (Lohr and others 2002). Snags are best retained as groups to provide multiple snags for roosting and foraging. Hardwood snags are used predominantly used for foraging, while pine snags are more commonly used for nesting (Smith and others 2000). Thinnings and prescribed fires that open the understory and create snags are beneficial.
The habitat suitability model for red-headed woodpeckers contains seven parameters:
- successional age class
- snag density
- large (>20 cm d.b.h.) snag density
- sawtimber (>28 cm d.b.h.) tree density
- the occurrence of edge
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 data from Hamel (1992) on the relative value of various vegetation types and successional age classes as red-headed woodpecker habitat in the Southeast.
Red-headed woodpeckers rely heavily on snags for nesting, foraging, and roosting. We used a logistic function
to predict how habitat suitability varied with snag density (SI2). We assumed 500 snags/ha represented an upper threshold above which maximal suitability (suitability index score = 1.000) was achieved and 200 snags/ha represented a threshold below which sites were unsuitable (suitability index score = 0.000;
Because the snag density in SI2 includes all dead trees >2.5 cm d.b.h., we also included large (>20 cm d.b.h.) snag density (SI3) as a model factor. This additional requirement ensured snags suitable for nesting would be present in high quality habitats. We relied on data from Lohr and others (2002) to inform a logistic function
that linked habitat suitability to large snag density numbers
Red-headed woodpeckers breed in fairly open habitats with widely spaced large trees near openings (King and others 2007). Therefore, we included sawtimber tree density (SI4) and edge occurrence (SI5) as variables in the habitat suitability model. We assumed red-cockaded woodpecker habitat suitability was highest (suitability index score = 1.000) when sawtimber tree density ≤20 trees/ha and was lowest (suitability index score = 0.000) when sawtimber tree density exceeded 50 trees/ha
. We fit a logistic function
through these data points to quantify the relationship between sawtimber tree density and habitat suitability index scores. To identify edges we used a 7 × 7 pixel moving window to locate the transitions between either forest and non-forest landcovers or sapling-pole-sawtimber and grass-forb-shrub-seedling successional age class stands. We assigned edge habitats the maximal suitability index score (1.000) and discounted areas without edge (suitability index score = 0.100;
To calculate the overall suitability index, we determined the geometric mean of suitability index scores for forest structure attributes (SI1, SI2, SI3, and SI4) and multiplied this product by the suitability index score for edge occurrence (SI5).
Overall SI = ((SI1 * SI2 * SI3 * SI4)0.250) * SI5