Factors governing the distribution of Swainson's warbler along a hydrological gradient in great dismal swamp

Abstract

Due to extensive clearing of bottomland forest in the southeastern United States, Swainson's Warbler (Limnothlypis swainsonii) is restricted in many drainages to seasonally inundated buffer zones bordering rivers and swamps. This migratory species is especially vulnerable to flooding because of its ground foraging ecology, but little is known about patterns of habitat occupancy at wetland ecotones. I investigated the physiognomic and floristic correlates of habitat use along a subtle hydrological gradient in the Great Dismal Swamp, southeastern Virginia. Hydrology is the driving force influencing vegetation and the distribution of Swainson's Warbler in that habitat. Foraging and singing stations of territorial males were significantly drier and more floristically diverse than unoccupied habitat. There was scant evidence that the distribution and abundance of particular plant species, including giant cane (Arundinaria gigantea), influenced habitat selection. Instead, Swainson's Warbler seems to evaluate potential territories on the basis of multiscale physiognomic, hydrological, and edaphic characteristics. Territories were characterized by extensive understory thickets (median = 36,220 small woody stems and cane culms per hectare; range, 14,000-81,400/ha), frequent greenbriar tangles, deep shade at ground level, and an abundance of leaf litter overlying moist organic soils. Those sites occurred most frequently in relatively well-drained tracts of broad-leaf forest that had suffered extensive canopy damage and windthrow. Data suggest a preference for early successional forest in the current landscape or disturbance gaps in primeval forest. Because territories in otherwise optimal habitat are abandoned when flooding extends into the breeding season, it is recommended that the water table be maintained at subsurface levels from late March through September in natural areas managed primarily for this species. Direct and indirect environmental factors that influence the breeding biology of the warbler are summarized in an envirogram.