James P. Gibbs, College of Environmental Science and Forestry, State
University of New York, 1 Forestry Drive, Syracuse, NY 13210
Scott M. Melvin, Massachusetts Division of Fisheries and Wildlife,
Rte. 135, Westborough, MA 01581
Shawn K. Crowley, Department of Forestry and Wildlife Management, Holdsworth
Hall, University of Massachusetts, Amherst, MA 01003
We broadcast vocalizations of pied-billed grebe (Podilymbus podiceps), American bittern (Botaurus lentiginosus), least bittern (Ixobrychus exilis), Virginia rail (Rallus limicola), and sora (Porzana carolina) at 60 wetlands in Maine in 1989 and 1990 to derive standardized methods to monitor breeding populations of these species. All wetlands contained 0.1-125.0 ha of emergent vegetation. Broadcasts of tape-recorded calls at 1-10 stations per wetland between 1 May and 15 July improved detectability for all species by 93 to 1,320% over 15-min passive observation periods at the edge of each wetland. Detection rates at wetlands where target species were known to occur ranged between 0.56 (least bittern) and 0.86 (pied-billed grebe) per survey visit. Three visits to a wetland were adequate to determine presence or absence of all species with 90% certainty. Least bitterns, soras, and Virginia rails were detected primarily within 50 m of observers, while pied-billed grebes and American bitterns were detected up to 500 m away. Most detections were of birds heard and not seen. Responsiveness of each species varied in relation to seasonal chronology, time of day, wind, precipitation, and cloud cover.
We surveyed 123 emergent wetlands in Massachusetts and Maine during the 1991 and 1992 breeding seasons to determine if broadcasts of tape-recorded calls increased detection rates of pied-billed grebes, American bitterns, least bitterns, green herons (Butorides striatus), Virginia rails, and soras. Following a 3-minute passive listening period at each station, we broadcast 1 minute of calls for each species at 1-10 survey stations per wetland. Detection rates of Virginia rails and soras were greater during the conspecific call portion of the broadcast sequence than during passive listening periods. In addition, both rails exhibited a peak of response within the broadcast sequence during broadcast of conspecific calls. In contrast, both green heron and American bittern had higher detection rates during passive listening periods than during broadcasts of calls. However, due to small sample size and potential bias, we recommend further study of responses of American bitterns to broadcasts of taped vocalizations. Sample sizes of pied-billed grebe and least bittern were too small for analysis.
We suggest that regional populations of marsh birds can be monitored using standardized surveys along mini-routes comprised of 10-15 point-count stations per route. From 10-15 survey points can be done in a morning, either within 1 large or several smaller wetlands. In the northeastern United States, we recommend that broadcasts of tape-recorded vocalizations be used to increase detection rates of pied-billed grebes, least bitterns, Virginia rails, soras, and, perhaps, American bitterns. Except for wetlands that are small and easily accessible, surveys should be conducted by canoe. Surveys should be conducted between 0430 and 1000 h on days with calm or light winds and no steady precipitation. Randomly stratifying sampling effort between large, species-rich wetlands and wetlands that are small, of marginal habitat quality, were historically occupied by target species, or have been recently created, for example by beavers, may be desirable. This design would balance the need to maximize detection rates while surveying populations over a representative sample of available habitats. Finally, monitoring habitat changes along mini-routes would be important for interpreting population trends of marsh birds.