Restoration Potential: Because of extensive post-breeding dispersal (Cramp 1977), bitterns are able to colonize new habitats and persist as small, isolated populations. For example, the Eurasian bittern recolonized Great Britain in the 1940s, where it was extirpated in the 1870s, and now persists in six small populations (50 pairs total) located distantly from one another (Bibby 1981, Day 1981). Readily uses artificial wetlands created by impoundments at waterfowl refuges (Andrle and Carroll 1988, Gibbs et al. in press), a trait that could facilitate restoration of populations in regions where natural, inland freshwater wetlands have been destroyed (Connecticut, Rhode Island, central New York, New Jersey, and Maryland) or were scarce originally (central Pennsylvania and West Virginia). Also seems adaptable to a wide range of wetland habitats, ranging from margins of boreal lakes in Quebec (DesGranges and Houde 1989) to dense cattail marshes in New York (Andrle and Carroll 1988), and can thrive at wetlands of many types as long as suitable prey and adequate cover are available (Gibbs et al. in press). Information is too scarce, however, on the structure and potential growth rates of populations to speculate about the recovery potential.
Preserve Selection and Design Considerations: Wetland area is a consideration for preserve design because both occurrence (Brown and Dinsmore 1986) and abundance (Gibbs et al. in press, Gibbs and Melvin 1990) are greater on larger than smaller wetlands. Wetlands of 2.5 ha or more may support nesting; smaller wetlands may serve as alternate foraging sites (Gibbs and Melvin 1992). In Maine, inhabited wetlands from less than one to greater than 25 ha in size, but were more abundant in larger than smaller wetlands, and preferred impoundments and beaver-created wetlands to wetlands of glacial origin (Gibbs et al. in press, Gibbs and Melvin 1990). Eaton (1914) suggested that there were occurrences in New York at marshes of less than four ha. In Iowa, Brown and Dinsmore (1986) observed individuals only on wetlands of greater than 11 ha and suggested that occurrences at wetlands was possibly area-dependent. A minimum area of 2.5-5 ha is therefore suggested as being sufficient to support nesting activity (see Eaton 1910, Brown and Dinsmore 1986, Gibbs et al. in press). Smaller wetlands adjacent to large wetlands used for nesting may serve as important, alternate foraging sites for these birds, which are seen regularly flying between wetlands during the nesting season in Maine (J. Gibbs, pers. obs.). Vegetative features of wetland preserves should include a high diversity of vegetative life forms and an abundance of emergent vegetation well-interspersed with patches of open water and aquatic-bed vegetation. Water levels should be less than 10 cm deep (Fredrickson and Reid 1986). Retaining dense, woody riparian vegetation may provide a visual barrier that reduces human disturbance of nesting bitterns and also buffers a wetland ecosystem against upland runoff that may contain silt, pesticides, and other contaminants (Gibbs and Melvin 1992).
Management Requirements: Wetlands used for breeding need to be protected from chemical contamination, siltation, eutrophication, and other forms of pollution that harm these birds or their food supplies. Day and Wilson (1978) emphasized that merely preserving habitats for endangered populations of Eurasian bitterns is not adequate to ensure their longterm viability; management of vegetation and monitoring water quality at protected areas also is required.
Vegetative features of preferred habitats represent a particular stage of wetland succession. Wetland managers therefore need to periodically reverse vegetative succession while maintaining suitable habitats nearby to serve as alternate nesting areas during wetland manipulations. The concentration of both nesting and overwintering populations at protected and managed wetlands such as state and national wildlife refuges (Andrle and Carroll 1988, Root 1988) emphasizes the need to develop and implement beneficial habitat management procedures. Minor alterations of existing management schemes could greatly improve nesting habitat. Where littoral vegetation is scarce, moist soil plant management (Fredrickson and Taylor 1982) provides a cost effective method involving water-level manipulation to reestablish and promote growth of dense stands of emergent vegetation.
Complete drawdowns should be avoided so that populations of small fish, amphibians, and dragonfly larvae, which make up a large part of the diet, are conserved for the following season. Slow, rather than rapid, drawdowns emulate natural water recession patterns, concentrate foods, and help prevent oxygen depletion (Fredrickson and Reid 1986). Drawdowns can be used to create favorable water levels (less than 10 cm deep, Fredrickson and Reid 1986) for foraging. Liming and fertilizing dikes and adjacent fields can increase productivity and raise the pH of many nutrient-poor, acidic wetlands in the northeastern region (Jorde et al. 1989). Control of infestations of purple loosestrife and phragmites may improve habitats in many northeastern states.
Management Research Needs: Develop standardized methodologies for population monitoring in the northeastern U.S.
Conduct surveys to determine abundance and distribution in the northeastern U.S.
Monitor trends in populations and habitat availability in the northeastern U.S.
Conduct a detailed, autecological study that examines basic features of the breeding biology of the species, including diet, home range, mating systems, ability to renest, sources and rates of mortality in adults, juveniles, nestlings, and eggs, and juvenile dispersal patterns and philopatry. This information could be gathered through radio-telemetry and banding studies.
Evaluate habitat requirements, including the vegetative characteristics, water quality, and minimum area of wetlands used by nesting, migrant, and overwintering birds. Also, evaluate effects of riparian zone management on wetland use.
Identify migration routes, major stopover sites, and major overwintering areas.
Examine the effects of contaminants, parasites, diseases, predation, water pollution, acid rain, human disturbance, and severe weather on populations.
Develop wetland management strategies that benefit nesting and migrant birds. Because the distribution of nesting birds is closely tied to protected wetlands at state and national wildlife refuges in many parts of the Northeast, assess the long-term viability of these sites as core-breeding areas for regional populations.
Monitor contaminant levels in adults and eggs in industrialized and agricultural regions of the northeastern U.S.
Evaluate the effects of marsh invasion by phragmites and purple loosestrife.