Restoration Potential: Will colonize artificial wetlands created by surface-mining (Perkins and Lawrence 1985) and abandoned industrial settling ponds (Rickard et al. 1981). Readily uses artificial wetlands at managed impoundments such as at state and federal waterfowl refuges (Andrle and Carroll 1988; Gibbs et al., in press). Artificial impoundments are thought to have reversed population declines in some areas and led to local population increases in others (Bull 1974). Their ability to renest following nest loss and raise two broods per year, its relatively large clutch size (up to seven eggs), and its tolerance of a wide range of habitat conditions, suggest that, given a stable habitat base, management potential for populations is high (Gibbs and Melvin 1992).
Preserve Selection and Design Considerations: Minimum wetland area is an important consideration in preserve design. Grebes typically occur on wetlands above a minimum threshold size, although this "minimum area" varies regionally. Minimum area requirements appear to be as large as five ha in the Midwest and Northeast (Brown and Dinsmore 1986; Gibbs et al., in press; Gibbs and Melvin 1990), although sites in the Midwest as small as 0.5 ha are used occasionally (Faaborg 1976). If provided with suitable habitat, large breeding populations can be supported on a single, managed wetland, e.g., up to 107 pairs have nested on an 81-ha impoundment in Louisiana (Chabreck 1963).
Moderately deep (0.25-2 m), stable waters represent an important feature of habitats for nesting. Key physical features of breeding areas also include large areas of aquatic-bed vegetation and open water, which can serve as deepwater feeding sites, interspersed with dense growth of robust emergents, with some patches over 100 m from shorelines that can serve as predator-free nest sites. Protection from human disturbance is important, e.g., from boats whose wakes can flood nests and recreationists whose presence can disturb incubating birds. Preserves also should be protected from upland run-off that may transport silt and contaminants and thereby lower wetland productivity and reduce food supplies (Gibbs and Melvin 1992).
In Maine, occurred only in wetlands > five ha in size, and were more common on wetlands impounded by beavers (CASTOR CANADENSIS) or humans than in wetlands of glacial origin (Gibbs et al. in press, Gibbs and Melvin 1990). Wetlands used by grebes had, on average, more aquatic-bed (submerged and floating vegetation) and emergent vegetation than did unused wetlands. In Iowa, individuals were observed at only 44 seasonal and semi-permanent wetlands among > 500 ponds and lakes surveyed (Faaborg 1976). Used wetlands of intermediate size (0.6-7.0 ha), but seemed to avoid either smaller or larger wetlands (Faaborg 1976). Another Iowa study suggested that the occurrence was dependent on wetland area because grebes occurred regularly only at wetlands > five ha (Brown and Dinsmore 1986). Wetlands as small as 0.3 ha, however, were used in Manitoba (Nudds 1982). In eastern Washington, 80% of broods were found at potholes of only 0.4-2.0 ha (Yocum et al. 1958).
Management Requirements: In the northeastern U.S., preservation of large, deepwater fresh and brackish marshes is the most important management need; managed waterfowl impoundments often provide prime nesting and migration habitat, and future efforts to protect or create wetlands significant to waterfowl could benefit the grebe (Gibbs and Melvin 1992). Preservation of relatively large (> 10 ha) wetlands with an interspersion of dense, robust emergents, submergent vegetation, and open water, is the most urgent management need in the Northeast. Wetlands used for breeding also need to be protected from chemical contamination, siltation, eutrophication, and other forms of pollution that harm grebes or their food supplies. Vegetative features of preferred habitats represent a particular stage of wetland succession ("hemi-marsh" stage of Weller and Spatcher 1965). Wetland managers, therefore, need to periodically reverse vegetative succession and open up extensive stands of emergent vegetation while maintaining suitable habitats nearby to serve as alternate nesting areas during wetland manipulations. In the northeastern states, wetlands characterized as deep, fresh marshes are perhaps best suited for nesting. For states with substantially reduced populations (e.g., Rhode Island, Connecticut, and New Jersey), creation of nesting habitats may be necessary to restore viable populations. Managed impoundments could also bolster nesting populations in areas where marshlands were scarce originally (e.g., central Pennsylvania, central and western Virginia, and West Virginia).
Because grebes occur in many wetlands managed for waterfowl by state and federal agencies, there is ample opportunity for making minor alterations to existing management schemes to improve nesting habitat. For example, portions of dense stands of cattail and bulrush, which are often removed with cutting, burning, or flooding treatments to improve waterfowl habitat, should be retained as nesting sites. Maintaining stable water levels during the nesting season prevents flooding of nests (a major source of reproductive failure) and predator access. Heavily dependent on aquatic-bed vegetation (floating-leaved and submergent) and management activities that promote establishment and growth of such vegetation will benefit grebes (Gibbs and Melvin 1992). Manipulation of water levels (Fredrickson and Taylor 1982) provides a cost-effective method for establishing dense stands of emergent vegetation while retaining open-water areas preferred by grebes.
Given the dependence of nesting birds on Odonates and small fish (Wetmore 1924), complete drying during wetland drawdowns should be avoided to prevent die-offs of dragonflies, many of which overwinter in late instars rather than in drought-resistant eggs (Orians 1980), and fish. The presence of carp may significantly lower availability of invertebrate foods for grebes and other waterbirds. Liming and fertilizing dikes and adjacent fields can increase the productivity and raise the pH of nutrient-poor, acidic wetlands in the northeastern region. Floating nests are easily washed over and capsized by wave action, and thus large, motorized boats should be excluded from marshes with nesting grebes. Recreation activity should be restricted during the nesting season to avoid disturbance to incubating grebes (Gibbs and Melvin 1992).
Management Research Needs: From Gibbs and Melvin (1992): 1) Conduct surveys to better determine relative abundance and distribution in the Northeast. 2) Develop standardized survey techniques and implement programs to monitor trends in populations and habitat availability. 3) Conduct detailed studies of the floristic and structural composition of wetland vegetation, water levels and water quality, and minimum wetland area associated with the occurrence of grebes during nesting and migration. 4) Determine the effects of diseases, parasites, contaminants and weather. 5) Investigate effects of altering management strategies at wetland impoundments managed primarily for waterfowl in order to benefit grebes. 6) Monitor contaminant levels in adults and eggs in agricultural and industrialized regions. 7) Determine major migration stop-over sites and study over-wintering habitats and biology. 8) Evaluate the effects of invasion of phragmites (PHRAGMITES AUSTRALIS) and purple loosestrife (LYTHRUM SALICARIA).
