Common Denominators of Swainson's Warbler Breeding Habitat in Bottomland Hardwood Forest in the White River Watershed in Southeastern Arkansas

The most intensively studied breeding population of Limnothlypis swainsonii (Swainson's Warbler) is in the White River watershed of southeastern Arkansas. However, because vegetation sampling protocols employed at this site have been significantly different from those used elsewhere, it has been difficult for land managers to reconcile datasets across the species' range in order to construct consensus quantitative benchmarks for optimal breeding habitat in bottomland hardwoods. We used a standardized sampling protocol to compare the physiognomic and floristic characteristics of breeding territories at 2 sites in the White River watershed with comparable data from other populations in Arkansas, Mississippi, Louisiana, Florida, and Virginia. Breeding territories in the combined dataset for this rare migratory species varied substantially in successional stage, floristic diversity, hydrology, and management history. Visual screening provided by understory thickets of saplings, vine tangles, and shrubs emerged as the most important common denominator of breeding territories in bottomland hardwood forests across the warbler's breeding range. Basal area, abundance of trees in larger-diameter classes, and floristic diversity appear to have little direct influence on habitat selection across the species' range. Although warblers are often associated with Arundinaria spp. (canebrakes), some of the most robust breeding populations occur in cane-free areas. Land managers tasked with generating and sustaining prime breeding habitat should strive for high counts of small woody stems (>45,000/ha or 4.5/m2) in areas that are infrequently subjected to flooding. This benchmark can be achieved through periodic canopy thinning and agroforestry clearcutting.