Pied-billed Grebe Habitat Model
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Draft Date:
May 2001

Species:
Pied-billed grebe, Podilymbus podiceps podiceps

Use of Study Area Resources:
Pied-billed grebes breed throughout the study area.  They also make minor wintering use of Cape Cod, Massachusetts, but we did not attempt to map this. Pied-billed grebes breed in central/southern Canada and throughout the U.S., southward into South America and the Hawaiian Islands (Muller and Storer 1999). They winter from British Columbia through the southern half of the U.S., and southward through the eastern Caribbean.

Habitat Requirements:
Cover. Pied-billed grebes occur in the Northeast in "ponds, sloughs and marshes...and occasionally estuarine wetlands" (Gibbs and Melvin 1992). In Maine, wetlands used by pied-billed grebes have relatively intricate shoreline edge, greater areas of aquatic bed vegetation, ericaceous vegetation, and emergent vegetation than wetlands not used (Gibbs and Melvin 1992). Wetlands altered by humans and beaver are used more frequently than wetlands of glacial origin (Gibbs et al. 1991). Gibbs et al. (1991) also observed grebe occurrences in palustrine scrub shrub deciduous, and coniferous and palustrine forested wetlands. Muller and Storer (1999) observed that fresh or brackish waters may be used, so long as they are "nonmoving".  

Grebes are solitary and reclusive nesters, easily disturbed by human intrusion (Muller and Storer 1999), and nesting microhabitat is probably selected to provide easy escape, either by diving from the nest, or by taking flight, for which they require access to open water. Forbes et al. (1989, in Gibbs and Melvin 1992) characterized nest sites as being a greater distance from shore, and closer to deep open water than randomly chosen marsh locations. Nests are floating masses of plant material, both rotting and fresh, and often anchored to emergent vegetation (Gibbs and Melvin, 1992). Mud and plant material are added "as the season progresses and as nests slowly sink" (Fjeldsa 1975, in Gibbs and Melvin 1992). The mud and rotting vegetation can keep the nest 11-13 degrees C warmer than the surrounding water (Davis et al 1985 in Gibbs and Melvin 1992). Hatchlings are vulnerable to death by exposure or chilling during the first two weeks post-hatch (Muller and Storer 1992). In Manitoba, Sealy (1978) found that water depths averaged 35.5 cm at nests and no nests were in water more shallow than 12.7 cm. Nests (n=31) averaged 5.3 m (+/- 2.4) from shore, and 1.3 m (+/- 0.9) from open water.

Nests may be threatened by wave action and fluctuations in water level (Glover 1948). Wakes from motor-boats, water-skiers, and even non-motorized boat traffic can destroy nests, or cause eggs to be lost (Muller and Storer 1992). Grebes in Nova Scotia avoided sites with emergent vegetation exposed to wave action (Forbes et al. 1989 in Gibbs and Melvin 1992).

Area. Pied-billed grebes were not observed on Maine wetland complexes < 5 ha (Gibbs et al. 1991); sites used typically had about 34 % open water. Brown and Dinsmore (1986) observed grebes at progressively greater frequencies in Iowa marshes as size increased from < 1 ha to > 20 ha.  Muller and Storer (1999) noted a minimum area of 0.2 ha for ponds used by grebes.  In the prairie pothole region of North Dakota and in eastern Washington, pied-billed grebes nested in ponds ranging from 0.6 to 7 ha (Faaborg 1976). It should be noted that prairie potholes vary greatly in size during the season, and most are ephemeral, so an acceptable pond size at initiation of nesting may be barely adequate by time nestlings have fledged. Gibbs and Melvin (1992) recommended preservation of large wetlands (> 10 ha) with an interspersion of emergent, submerged aquatic vegetation, and open water as breeding habitat.

Food. Pied-billed grebes feed opportunistically on the most readily available prey, feeding more on fish in the winter. In addition to fish, they feed on crustaceans (mostly crayfish), and insects, especially dragonfly/damselfly larvae, and beetles (Wetmore 1924 in Gibbs and Melvin 1992). They forage underwater by diving, and frequently sunbathe between dives, which may be a means of conserving and regulating body temperature (Gibbs and Melvin 1992).

Occurrence Data within the Study Area:
The New Hampshire Natural Heritage program supplied point data for 10 occurrences of pied-billed grebes, and these were used to verify or supplement information obtained from the literature. In particular, comparison of the photo-interpreted and satellite landcover with grebe nesting locations indicated that used areas may include palustrine deciduous forested and shrub, palustrine coniferous shrub, and palustrine emergent cover types. The minimum open water area associated with occurrences was 1.53 ha, and the minimum overall patch size of used wetland complexes was 13.4 ha.

Model:
We regarded estuarine and riparian waters as generally unsuitable because of excessive motion (flow).  Grebe habitat was mapped by regarding fresh water bodies and vegetated wetlands as complementary habitat components: open water and aquatic bed vegetation constituted one component, while emergent, wet forest or shrub vegetation were another (see table, below). If both components were present anywhere within an aquatic/wetland complex > 5 ha in area, it was scored 0.5. Where both components were present within a 4-cell window (120 by 120 m), and the overall aquatic/wetland complex was > 5 ha, they were score 1.0. Finally, open water in the centers of lakes (more than 300 m from shore or from the wetland vegetation component) was deleted.

NWI Designations (wetlands only)	Cover Types	Cover Suitability (0 - 1 scale)
	Upland deciduous forest
	Upland coniferous forest
	Upland mixed forest
	Grassland
	Upland scrub/shrub
	Cultivated
	Developed
	Bare ground
PEM, L2EM	Lake/pond, emergent vegetation	1.0*
PFOcon	Palustrine forest, conifer
PFOdec	Palustrine forest, deciduous	1.0*
PSSdec	Palustrine scrub shrub, deciduous	1.0*
PSScon	Palustrine scrub shrub, conifer	1.0*
PAB, L2AB	Lake/pond, aquatic vegetation	1.0**
L1UB, PUB	Lake/pond, unconsolidated bottom	1.0**#
L2US	Lake, unconsolidated shore	1.0**
L2RS	Lake, rocky shore	1.0**
R1UB	Riverine subtidal unconsolidated
Rper	Riverine perennial
E1AB	Estuarine subtidal vegetated
E1UB	Estuarine subtidal unconsolidated bottom
E2AB	Estuarine intertidal algae
E2EM	Estuarine intertidal emergent
E2RS, R1RS	Estuarine, tidal river rocky shore
E2SS	Estuarine intertidal shrub
E2US	Estuarine intertidal unconsolidated shore
M1AB	Marine subtidal vegetated
M1UB	Marine subtidal unconsolidated bottom
M2AB	Marine intertidal algae
M2RS	Marine intertidal rocky shore
M2US	Marine intertidal unconsolidated shore
NOTES	* Used as vegetated part of a water/wetland complex **Used as aquatic part of water/wetland complex #Used if within 300 m of either shore or vegetated components

Model testing: Occurrence information given in the appendices of Gibbs et al. (1991) were used to test the grebe model. We compared the distribution of modeled habitat around a random set of 798 upland points to that for the ponds and wetlands at which grebes had been observed in that study Of 16 sites with birds, 14 had mapped habitat, while 176 sites out of the 798 randomly distributed sites had habitat. The Chi-square was highly significant, indicating that the overall model does indicate localities useful to grebes.

Maine Department of Inland Fisheries and Wildlife marsh bird survey data (courtesy of T. Hodgman) also were used to test the habitat map. We compared the distribution of mapped habitat around a random set of 798 upland points to that for marsh bird survey stops at which grebes were observed in 1998 through 2000. Of the 9 sites with birds, 8 had mapped habitat, while only 58 sites out of the 798 randomly distributed sites had habitat. This Chi-square also was highly significant. The higher scored habitats (1.0) from the 'window' analysis gave an even stronger level of significance, indicating that this assignment was appropriate.  

Sources:
Brown, M. and J.J. Dinsmore. 1986. Implications of marsh size and isolation for marsh bird management. J. Wildl. Manage. 50(3):392-397.

DeGraaf, R.M. and D.D. Rudis. 1983. New England Wildlife: Habitat, Natural History and Distribution. USDA Technical Report NE-108.

Faaborg, J. 1976. Habitat selection and territorial behavior of the small grebes of North Dakota. Wilson Bull. 88(3):390-399.

Gibbs, J.P., J.R. Longcore, D.G. McAuley and J.K. Ringelman. 1991. Use of wetland habitats by selected nongame water birds in Maine. U.S. Fish Wildl. Serv., Fish Wildl. Res. 9. 57 pp.

Gibbs, J.P. and S.M. Melvin. 1992. Pied-billed grebe. Pp. 31-49 in K.J. Schneider and D. Pence, eds. Migratory nongame birds of management concern in the northeast. U.S. Fish Wildl. Serv. 400pp.

Glover, F.A. 1948. Nesting ecology of the pied-billed grebe in northwestern Iowa. The Wilson Bull. 65(1):32-39.

Muller, M.J. and R.W. Storer. 1999. Pied-billed grebe (Podilymbus podiceps). In A. Poole and F. Gill (eds.) The Birds of North America, No. 410. The Birds of North America, Inc., Philadelphia, PA.

Sealy, S.G. 1978. Clutch size and nest placement of the pied-billed grebe in Manitoba. Wilson Bull. 90(2):301-302.