The great crested flycatcher is an obligate cavity nester in deciduous forest habitats of the eastern United States; it is generally absent in pure evergreen stands (Lanyon 1997). The species is not area-sensitive but does require a minimum amount of forested habitat in the landscape. Great crested flycatchers may nest in patches as small as 0.2 ha and their abundance may actually decline in forest interiors (Robbins and others 1989). Great crested flycatchers do not occur in riparian corridors surrounded by <14.7 percent forest (Perkins and others 2003b), and detection probabilities steadily increase with increasing riparian corridor width (Groom and Grubb 2002).
The great crested flycatcher forages by sallying from exposed perches (Lanyon 1997). Thus, open forest stands are preferred. Holmes and others (2004) found highest great crested flycatcher abundance in heavily cut stands where a third or more of the basal area had been removed. Similarly, Moorman and Guynn (2001) found great crested flycatchers associated with large (0.5 ha) canopy gaps in bottomland hardwood forest in South Carolina. Snags provide not only exposed perches for foraging but also cavities for nesting and great crested flycatchers are negatively affected by the removal of snags associated with some forestry practices (Lohr and others 2002). Where snags are lacking, great crested flycatchers will use nest boxes and other artificial cavities, which enable them to occupy cemeteries, suburban parks, and wooded pastures. Wakeley and Roberts (1996) found great crested flycatchers associated with mesic sites, but this may reflect a preference for bottomland hardwoods over evergreen uplands in the Southeast.
The habitat suitability index model for great crested flycatcher contains six parameters:
- successional age class
- snag density
- distance to edge
The first suitability function combines landform, landcover, and successional age class into a single matrix (SI1) defining unique combinations of these classes
. We directly assigned suitability index scores to these combinations based on the relative habitat quality associations reported in Hamel (1992) for great crested flycatchers.
Great crested flycatchers rely on snags (SI2) for nesting and foraging. We fit a logistic function
to data from Lohr and others (2002) demonstrating the effect of manipulated snag densities on great crested flycatcher abundance
Great crested flycatchers are associated with edges, and their abundance declines with increasing distance from an edge (SI3). Small and Hunter (1989) observed more than 60 percent of all flycatchers <60 m from an edge. We assumed maximum habitat suitability (i.e., suitability index score = 1.000) at the edge and modeled the relationship between distance to edge and suitability index score as an inverse logistic function through these data points
To calculate the overall suitability index, we determined the geometric mean of SI scores for forest structure (SI1 and SI2) and then calculated the geometric mean of this value with the edge function (SI3).
Overall SI= ((SI1 * SI2)0.500 * SI3)0.500
Great crested flycatchers occurred in all 88 subsections within the CH and WGCP. Spearman rank correlation on average HSI score and mean BBS route abundance failed to identify a significant (P ≤ 0.001) association (r = 0.55) between these two variables. Similarly, negative binomial regressions at both the subsection and route levels did not demonstrate any positive relationship between HSI scores and eastern field sparrow abundance. We considered this model neither verified nor validated.