The swallow-tailed kite is rare breeder in the continental United States. The current restriction of swallow-tailed kites to seven southern states (with limited distributions in all but Florida) represents a significant contraction of the former range of this species. Most information on this species in the United States comes from Florida (Meyer 1995).
Swallow-tailed kites have large home ranges (500–1800 ha) that are substantially increased (>20000 ha) if the long but regular foraging forays characteristic of this species are included. With such a large home range, the importance role of landscape structure on habitat suitability is not surprising. Critical habitat elements are large tall trees for nesting and open habitats containing prey (Meyer 1995, Sykes and others 1999). Any interspersion of these features is usable (e.g., trees adjacent to prairie, wetlands, or marsh). Landscapes containing bottomland hardwood forest interspersed with scattered openings are particularly attractive. The edges of pine forests along swamps and riparian zones are also commonly used along the Coastal Plains. Mississippi kite habitat is typically drier and contains more contiguous forest than the habitats utilized by swallow-tailed kites.
The habitat suitability model for swallow-tailed kites includes six parameters:
- successional age class
- forest patch size
- landscape composition
- dominant tree density
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 directly assigned suitability index scores to these combinations based on relative habitat quality rankings from Hamel (1992) for swallow-tailed kites. However, we assumed only stands in the sawtimber successional age class provided suitable habitat for swallow-tailed kites
We also included forest patch size (SI2) as a parameter because of this species’ large home range and association with large blocks of forested wetlands. We fit a logarithmic function
to data adapted from a variety of sources
to assess the impact of forest patch size on habitat suitability scores for swallow-tailed kite habitat.
Like Mississippi kites, swallow-tailed kites forage aerially in open habitats. Therefore, they require both forested sites for nesting and open areas for foraging (SI3). We based the ideal composition of vegetation types in the landscape on data from Sykes and others (1999), who observed ~20 percent open habitat within 200-ha core areas of swallow-tailed kites in Florida. We maximized swallow-tailed kite habitat suitability at this threshold and reduced their abundance as landscapes contained more or less open habitat
Swallow-tailed kites nest in dominant trees (SI4) that extend above the canopy. We assumed trees with a d.b.h. >76.2 cm (30”) would extend above the canopy in the sawtimber stands that provide the exclusive habitat for this species. We assumed 1 dominant tree/ha would satisfy this requirement and that swallow-tailed kites would be absent from stands with a uniform canopy (i.e., 0 dominant trees/ha). We fit an exponential function
to the values between these data points and assumed stands with ≥14 dominant trees/ha (the maximum value from the WGCP in the 1992 FIA data) were associated with maximum swallow-tailed kite habitat suitability
To calculate the overall suitability index score, we determined the geometric mean of suitability index scores for forest structure attributes (SI1 and SI4) and landscape composition (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