American Bittern Habitat Model
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Draft Date:
March 2001

Species:
American bittern, Botaurus lentiginosus

Use of Study Area Resources:
Reproduction, migration: American bitterns breed in most states in the northern half of the continental U.S., and provinces of the sub-arctic/southern half of Canada (Gibbs et al. 1992). They winter along the coastal plain of the south Atlantic, the Gulf Coast, southern California and Mexico (Gibbs et al. 1992, DeGraaf and Rappole 1995). Smaller numbers winter in the Bahamas, Cuba, Greater Antilles, Lesser Antilles, and rarely in Puerto Rico (Degraaf and Rappole 1995).

Habitat Requirements:
The American bittern prefers wetlands that provide both feeding and nesting resources (Gibbs and Melvin 1992). They use predominantly freshwater wetlands with vegetation that provides protective cover and hosts a forage base of insects, small fish, amphibians, and small mammals (Gibbs et al. 1992).  Bittern habitat typically is dominated by tall emergent or aquatic bed vegetation with a high degree of cover-water interspersion. This includes wetland fringes, shorelines, bogs, swamps, wet meadows, but rarely tidal marshes (Gibbs et al. 1992, Gibbs and Melvin 1992, DeGraaf and Rappole 1995). Nests are built on the ground or on tussocks of emergent vegetation surrounded by water (Gibbs and Melvin 1992).

American bitterns are sighted most frequently in palustrine emergent, scrub-shrub, and aquatic bed wetlands (Gibbs et al. 1991), but occasionally are seen in coastal marshes in Maine (Pierson et al. 1996). They use wetlands of all sizes (0.01 - 1,000 ha). Gibbs et al. (1991) observed the abundance of bitterns in Maine was positively associated with wetland area.  Studies in Maine, New York, and Iowa suggested that American bitterns will nest only on wetlands of 2.5 to 11 ha or larger ( Gibbs et al. 1991, Gibbs and Melvin 1992, Gibbs et al. 1992). Human disturbance may seriously reduce habitat quality (Gibbs and Melvin 1992), but Gibbs et al. (1991) did not find a significant association between distance from roads and whether wetlands were used or unused.

American bitterns may be affected by direct loss or by degradation of habitat. The latter includes: human disturbance, which can interfere with foraging; declines in water quality which can change vegetative composition and structure; invasion by exotic species, such as purple loosestrife or Phragmites which may reduce the abundance and diversity of species useful to bitterns and their prey (Gibbs and Melvin 1992).

Model:
Habitat was mapped by scoring wetland cover types, considering the interspersion of vegetation and water, and adjusting scores for size of habitat patches and proximity to development.

Interspersion: suitable emergent and shrub wetland types (E2EM, PEM, PSS) and grasslands which were located within 30 m of either open water or aquatic bed vegetation (PAB, L2AB) were scored at the "cover suitability" values given in the table below.  We included as open water both features from National Wetlands Inventory maps and stream features from USGS 1:100000 hydrology.  Aquatic bed vegetation within 30 m of emergent vegetated or shrub wetland types or grassland, also was given a score equal to its indicated "cover suitability".

Sites more than 30 m from the complementary cover types were scored at half the nominal value, except that grassland cover over 30 m from water or aquatic bed vegetation was scored 0.

NWI Designations (wetlands only)	Cover Types	Cover Suitability (0 - 1 scale)
	Upland deciduous forest
	Upland coniferous forest
	Upland mixed forest
	Grassland	0.5*
	Upland scrub/shrub
	Cultivated
	Developed
	Bare ground
PEM, L2EM	Lake/pond, emergent vegetation	1.0*
PFOcon	Palustrine forest, conifer
PFOdec	Palustrine forest, deciduous
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
L2US	Lake, unconsolidated shore
L2RS	Lake, rocky shore
R1UB	Riverine subtidal unconsolidated
Rper	Riverine perennial
E1AB	Estuarine subtidal vegetated
E1UB	Estuarine subtidal unconsolidated bottom
E2AB	Estuarine intertidal algae
E2EM	Estuarine intertidal emergent	0.4*
E2RS, R1RS	Estuarine, tidal river rocky shore
E2SS	Estuarine intertidal shrub
E2US, R1US	Estuarine, riverine 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	*score if within 30 m of open water or aquatic bed vegetation **score if within 30 m of emergent or shrub wetland types, or grassland

Patch size: habitat patches were re-scored based on area suitability. We first examined the distribution of patch sizes associated with bittern occurrences along Breeding Bird Survey routes in 1990, 1997 and 1998 throughout the study area.  In half of 24 Breeding Bird Survey locations habitat patch size was 12 cells (1 ha) or smaller.  Based on this finding, and on those of Gibbs et al. (1991), we regarded such smaller patches as suitable (0.7), and larger patches as optimal (1.0).

Proximity to development: habitat at least 90 m from development/cultivated land was scored 1.0; if within 90 m the score was reduce by half.

Overall Habitat Score = cover suitability x interspersion suitability x area suitability x disturbance suitability

Model testing: Maine Department of Inland Fisheries and Wildlife marsh bird survey data (courtesy of T. Hodgman) 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 bitterns were observed in 1998 through 2000. Of the 99 sites with birds, 92 had mapped habitat, while only 71 sites out of the 798 randomly distributed sites had habitat. The Chi-square was highly significant, indicating that the overall model predicts localities useful to bitterns.  Restricting the test to habitats scored above 0.5 gave even an higher probability of association, supporting our premise that more highly scored areas have a higher suitability for this species.

Sources:
DeGraaf, R.M. and J.H. Rappole. 1995. Neotropical Migratory Birds: Natural History, Distribution and Population Change. Comstock Publishing Associates, Ithaca, NY. 1995. 676 pp.

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

Gibbs, J.P., S. Melvin and F.A. Reid. 1992. American Bittern, Botaurus lentiginosus. In A. Poole and F. Gill, (eds.) The Birds of North America, No. 18. The Academy of Natural Sciences, Philadelphia, and The American Ornithologists' Union, Washington, D.C.

Gibbs, J.P. and S. Melvin. 1992. American Bittern. Pp 51-88 in Schneider, K. J. and D. M. Pence (eds.) Migratory Nongame Birds of Management Concern in the Northeast. 1992 U.S. Fish and Wildlife Service, Newton Corner, MA. 400p.

Pierson, E.C., J E. Pierson, and P.D. Vickery. 1996. A Birders Guide to Maine. Down East Books, Camden, ME.