Grasshopper Sparrow Habitat Model
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Draft Date:
May 2001

Species:
Grasshopper sparrow, Ammodramus savannarum

Use of Study Area Resources:
Reproduction. Grasshopper sparrows reach the northern extent of their breeding range in Massachusetts and the southern regions of Maine and New Hampshire (Shriver et al. 1999). They winter from the mid-Atlantic states through the Gulf Coast and into Mexico, as well as the West Indies (Vickery 1996). 

Habitat Requirements:
Cover. Grasshopper sparrows utilize prairie and cultivated grasslands, weedy fallow fields, and alfalfa fields. They avoid significant shrub cover (DeGraaf and Rappole 1995, Vickery 1996). They occupy intermediate grassland habitat, preferring drier sparse sites in tallgrass prairies, with open or bare ground for feeding (Vickery et al. 1992, Vickery 1996). In Maine grasshopper sparrows occur also in blueberry barrens, airfields, and hayfields (Boone, Pierson et al. 1996). Grasshopper sparrows are vulnerable to early mowing of fields, while light grazing, infrequent and post-season burning or mowing can be beneficial (Vickery 1996). With few exceptions, nests are built on the ground, near a clump of grass or base of a shrub, "domed" with overhanging vegetation (Vickery 1996).

Area. Grasshopper sparrow reproductive habitat is generally > 10 ha in extent (DeGraaf and Rappole 1995). The optimum habitat area in Maine is about 100 ha (Vickery et al. 1994), although in Michigan they were historically found in some sites of only a few hectares (Walkinshaw 1940 in Vickery 1996). In Illinois the minimum area used was 10 - 30 ha (Herkert 1991 in Johnson et al. 1998), and 8 ha in Nebraska (Helzer 1996 in Johnson et al. 1998).

Interspersion and edge effects. Nest parasitism was found to be high adjacent to woody edges (within 45 m) in Minnesota, especially in smaller grasslands (Johnson and Temple 1990 in Johnson et al. 1998); sparrow densities also were lower within 75 m of wooded edges (Helzer 1996 in Johnson et al. 1998).  Grasshopper sparrow densities also were lower within 200 m of development in Colorado (Bock et al. in press, in Johnson et al. 1998).

Foraging. Insects represent most of the summer diet, especially grasshoppers, with the remainder comprised of small seeds. Dietary proportions of seeds and insects change as seasonal availability changes; winter diet is primarily seeds, especially from grasses and sedges (Vickery 1996).

Threats. Population declines in the East are attributed to habitat reductions from suburban development, intensive agriculture, and forest succession (Vickery 1996). Grasshopper sparrow nests are vulnerable to early mowing of fields, while light grazing, infrequent and post-season burning or mowing can be beneficial (Vickery 1996). They are also susceptible to predation by hawks, loggerhead shrikes, small mammals, snakes, and parasitism by brown-headed cowbirds (Vickery 1996).

Management. Within the study area, grasslands associated with airports have provided important habitat. Recommended management includes deferring mowing until August, except within 10 m of runways. The taller grass cover is beneficial to grasshopper sparrows and less attractive to roosting by gulls or crows, both of which are a hazard to aircraft. (Vickery 1996).

Model:
The grasshopper sparrow range within the study area was delineated by selecting all USDA Forest Service Ecological subunits (Keys et al. 1995) in which the sparrows were known to occur. Subsequent mapping and testing of the model was restricted to these areas. Suitable land cover types (see table, below) were selected, and their suitability scores adjusted according to their distance from development and forests. Habitat within 180 m (6 30-m cells) of development or 60 m of forest were regarded as having half (0.5) the suitability value of interior grasslands.

The size or extent of habitat patches then was considered: patches over 100 ha were regarded as optimal (1.0); patches between 30 and 100 ha half that suitability (0.5); those from 8 to 30 ha were scored 0.3, while smaller patches were regarded as of no value. These patch size scores were combined with the above development/forest adjusted scores as a geometric mean, yielding the overall habitat suitability scores.

Model testing: The grasshopper sparrow occurrences from the Shriver et al. (1999) database and Maine Department of Inland Fisheries and Wildlife heritage database were used to test the habitat map. We had used the former to score the relative value of habitat clusters, but not to delineate habitat, and so they still were eligible for testing presence/absence. We compared the presence of habitat near a random set of 322 upland points within the range of the grasshopper sparrow to the presence of habitat at sites where sparrows actually were observed. Of the 18 sites with birds, 12 had mapped habitat, while 106 sites out of the 322 randomly distributed sites had habitat. The Chi-square was significant (P = .002), indicating that the overall model does indicate localities useful to grasshopper sparrows. Mapped habitat distribution also was checked against Pierce and Melvin (1991) and Weik (1999) surveys of major grassland habitats in Maine to confirm the overall habitat rankings.

Sources:
Boone, R. Maine Gap Analysis, Maine Cooperative Fish and Wildlife Research Unit; www site http://www.wle.umaine.edu/progs/unit/gap/

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

Johnson, D.H., L.D. Igl, J.A. Dechant, M.L. Sondreal, C.M. Goldade, M.P. Nenneman and B.R. Euliss. 1998. Effects of management practices on grassland birds: grasshopper sparrow. Northern Prairie Wildlife Res. Center, Jamestown, ND. 12 pp.

Keys, J.E., Jr., J.C. Carpenter, S. Hooks, F. Koenig, W.H. McNab, W. Russell and W. Smith. 1995. Ecological units of the eastern United States - first approximation (map and booklet of map unit tables), USDA Forest Service. Atlanta, GA.

Pierce, S.P. and S.M. Melvin. 1991. Assessment of barrens/grassland birds and habitats in Maine. Report to Maine Department of Inland Fisheries and Wildlife, Augusta, ME. 14 pp.

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

Shriver, W.G., R.J. MacCulloch and J.V. Wells. 1999. Grassland Birds Data Compilation Project for the Northeast U.S. (USFWS Region 5). USFWS, Hadley, MA.

Vickery, P.D. 1996. Grasshopper Sparrow. The Birds of North America, 239.

Vickery, P.D., M.L. Hunter, Jr. and J.V. Wells. 1992. Use of a new reproductive index to evaluate relationship between habitat quality and breeding success. Auk 109:706-710.

Vickery, P.D., M.L. Hunter, Jr. and S.M. Melvin. 1994. Effects of habitat area on the distribution of grassland birds in Maine. Conservation Biology 8(4):1087-1097.

Weik, A. 1999. 1999 Annual Report, Grasshopper Sparrow Breeding Bird Census. Maine Department of Inland Fisheries and Wildlife, Bangor, ME. 3 pp.