Distribution, Abundance, and Habitat Affinities of the Coastal Plain Swamp Sparrow

—We examined the distribution and abundance of the Coastal Plain Swamp Sparrow (Melospiza georgiana nigrescens) at previously occupied sites and points within potential habitat. We found Swamp Sparrows throughout their formerly documented range except in southern Chesapeake Bay. Swamp Sparrows were most common in the Mullica River region of New Jersey where we detected individuals at 78% of systematically chosen points with a mean count of 4.1 birds/point. The percentages of points with positive detections in the regions of Delaware River (39%), eastern Delaware Bay (23%), western Delaware Bay (34%), and Tuckahoe River (31%) were lower. The mean count of birds/point was between 0.4 and 0.6 in these regions. A higher resolution Poisson model of relative abundance suggested that the greatest concentrations of Swamp Sparrows occurred not only in the Mullica River area but also along northwestern Delaware Bay. Regression analysis of Swamp Sparrow counts and habitat features identified shrubs (Iva frutescens and Baccharis halimifolia) as a key habitat component. By applying density estimates generated by DISTANCE (Thomas et al. 1998) to the approximate area of potential shrub habitat along Delaware Bay, we estimated that the core population of Coastal Plain Swamp Sparrows was less than 28,000 pairs. We recommend that the Coastal Plain Swamp Sparrow be listed as a subspecies of concern by state and local governments because of its relatively small population size, restricted distribution in the mid-Atlantic region, and narrow habitat requirements. Received 23 April 2002, accepted 13 November 2002. The Coastal Plain Swamp Sparrow (Melospiza georgiana nigrescens) is one of three subspecies of Swamp Sparrow, but unlike the more common races, M. g. nigrescens is restricted to a small region of the Mid-Atlantic coastal bay ecosystem. Originally described from specimens collected in the Nanticoke River marshes during 1947 (Bond and Stewart 1951), M. g. nigrescens occurs only in estuarine marshes of the upper Chesapeake and Delaware bays and in large river drainages along the Atlantic coast of New Jersey north to the tidal portions of the Hudson River (Greenberg and Droege 1990). Subsequent analysis has shown that M. g. nigrescens is morphologically distinct from all other Swamp Sparrows (Greenberg and Droege 1990). The nigrescens race is one of the few vertebrate subspecies endemic to the mid-At1 U.S. Geological Survey, Patuxent Wildlife Research Center, 12100 Beech Forest Rd., Laurel, MD 20708-4038, USA. 2 Migratory Bird Center, National Zoological Park, Washington, DC 20008, USA. 3 U.S. Fish and Wildlife Service, Div. of Migratory Bird Management, Laurel, MD 20708, USA. 4 Current address: Molecular Genetics Lab., Smithsonian Inst., 3001 Connecticut Ave. NW, Washington, DC 20008, USA. 5 Corresponding author; e-mail: [email protected] lantic region, but investigations into its biology and status are lacking. Within mid-Atlantic estuaries, Swamp Sparrows (M. g. nigrescens unless otherwise noted) are restricted to an often narrow band of shrubby habitat at the interface of upland and high marsh. This habitat is particularly susceptible to recent changes such as rising sea level, ditching, impoundment, the invasion of the reed Phragmites australis, and the development of roads, farm fields, and housing tracts. Studies conducted during the late 1980s identified breeding populations of up to 70 singing males at several sites in the upper Chesapeake Bay (RG unpubl. data). Subsequent visits to these and other sites with formerly documented breeding populations revealed marked reductions of Swamp Sparrow numbers in Maryland. Given significant declines of similar marsh-nesting subspecies, including the Cape Sable Seaside-Sparrow (Ammodramus mirabilis; Pimm et al. 1996), Dusky Seaside-Sparrow (A. nigrescens; Sykes 1980), and San Francisco Bay Song Sparrow (Melospiza melodia spp.; Marshall and Dedrick 1994), we undertook a standardized survey of the Chesapeake and Delaware bays to examine the current status of Coastal Plain Swamp Sparrows. We surveyed previously occupied sites and sites within potential 39 Beadell et al. • COASTAL PLAIN SWAMP SPARROW breeding habitat to evaluate range changes, characterize habitat associations, identify regions of potential management importance, and generate baseline population estimates.