Northern Harrier Habitat Model
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Draft Date:
May 2001

Species:
Northern harrier, Circus cyaneus

Use of Study Area Resources:
Reproduction and wintering: Maine, New Hampshire and Massachusetts are all within the breeding range for northern harriers. Massachusetts, southern portions of Maine and New Hampshire offer wintering habitat (Bent 1937 and Terres 1980 in Serrentino 1992).

Habitat Requirements:
Nesting. Breeding habitat may include open wetlands, wet meadows, pastures, old fields, freshwater and brackish marshes, grasslands, agricultural fields, shrublands and riparian corridors (MacWhirter and Bildstein 1996). Depending on the location, harriers will nest in either dry or wetland sites. Harriers used cattail and marsh grasses in New Brunswick in preference to drier sites (Simmons and Smith 1985 in Johnson et al. 1998), while harriers nesting on the coastal plains of Nantucket Island (Massachusetts) most often chose upland sites with dense vegetation and shrubs 0.5 meter or less in height (Combs-Beattie 1993). The latter may represent a case where preferred habitats are unavailable (F. Hamerstrom 1986 in Christiansen and Reinert 1990, Hamerstrom and Kopeny 1981). Habitat has reduced value where tall shrubs (>2 m) or trees may host predators (Brian Toland, Florida Game and Fresh Water Fish Comm., pers. comm.).

Foraging, Roosting and Wintering. Harriers typically forage on the wing, gliding low over fields and marshes for small mammals, birds, and occasionally frogs or snakes, shifting their prey base with the seasons (MacWhirter and Bildstein 1996). Harriers form communal (winter) roosts on open ground, ranging from several to 60 birds (Bildstein 1979). Wintering habitat is similar to breeding habitat, with more emphasis on dry upland areas such as dunes, coastal grasslands, croplands, fallow fields and floodplains (MacWhirter and Bildstein 1996). Ground roosts may be abandoned for forest roosts during periods of deep snow or flooding; harriers occasionally roost with short-eared owls (Bildstein 1979, John B. Holt, Jr. pers.comm.).

Area Requirements. Minimum patch size for nesting may range between 8 ha and 16 ha (Herkert et al. 1999, in southeastern Illinois). Serrentino (1992), working in Massachusetts, found harriers nesting in fields of 11 ha to 54 ha, while in Maine blueberry fields, harriers were seen only at sites over 100 ha (Vickery et al. 1994).  They prefer to forage on wetlands 1 ha or larger, particularly those with emergent vegetation or openwater (Gibbs et al. 1991).

Sensitivity to Disturbance. Harriers prefer undisturbed grasslands for nesting (Herkert et al.1999); they will use lightly grazed sites, but such nests are at risk of being trampled (Hamerstrom 1969; Toland 1986 in Herkert 1999; MacWhirter and Bildstein 1996). Nests or young may be lost to harvesting of early crops, haying or tilling (Craighead and Craighead 1956, Hamerstrom 1969), but nesting birds may tolerate agricultural activities if they are in adjacent areas (Serrentino 1992). Nests on Nantucket Island were 188 m or more from the nearest buildings (n = 9, Combs-Beattie 1993). Construction or other human activities often cause harrier abandonment of nests or roosts (Brown and Amadon 1968, Newton 1979).

Model:
Habitat was mapped by selecting appropriate cover types (see table, below), and adjusting the scores for patch size and distance from development. We eliminated patches smaller than 10 ha in area, and reduced habitat value by half for sites that were within 180 m of development.
NWI Designations
(wetlands only)
Cover Types Cover Suitability
(0 - 1 scale)
Upland deciduous forest
Upland coniferous forest
Upland mixed forest
Grassland 1.0
Upland scrub/shrub *
Cultivated

0.5

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
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 1.0
E2RS, R1RS Estuarine, tidal river rocky shore
E2SS Estuarine intertidal shrub 1.0
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 *the upland scrub/shrub category of our data set is dominated by clearcut and early regenerating forest, not suitable structures for harriers

Model testing: The northern harriers occurrences from Breeding Bird Survey routes, grassland bird survey database (Shriver et al. 1999), and New Hampshire heritage database were used to test the habitat map. We compared the presence of habitat near a random set of 798 upland points to that for sites at which field sparrows were observed during 1990 through 1998. Of the 24 sites with birds, 20 had mapped habitat, while only 221 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 northern harriers.

Sources:
Bildstein, K.L. 1979. Fluctuations in the number of Northern Harriers (Circus cyaneus hudsonius) at communal; roosts in south central Ohio. Raptor Res. 13:40-46.

Brown, L.H. and D. Amadon 1968. Eagles, hawks, and falcons of the world. McGraw Hill, NY.

Combs-Beattie, K. 1993. M.S. Thesis: Ecology, Habitat Use Patterns and Management Needs of Short-eared Owls and Northern Harriers on Nantucket Island, Massachusetts. University of Massachusetts - Amherst.

Christiansen, D.A. Jr. and S.E. Reinert. 1990. Habitat use of the northern harrier in a coastal Massachusetts shrubland with notes on population trends in southeastern New England. J. Raptor Res. 24(4):84-90.

Craighead, J.J. and F.C. Craighead, Jr. 1956. Hawks, Owls and Wildlife. Dover Publications, Inc., NY. Pp 93-98.

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.

Hamerstrom, F. 1969. A harrier population study pp 367-383 in Peregrine Falcon populations, their biology and decline, J.J. Hickey, ed., Univ. of Wisconsin Press, Madison, WI.

Hamerstrom, F. and M. Kopeny. 1981. Harrier nest-site vegetation. Raptor Res. 15(3):86-88.

Herkert, J.R., S.A. Simpson, R.L. Westemeier, T.L. Esker and J.W. Walk. 1999. Response of northern harriers and short eared owls to grassland management in Illinois. J. Wildl. Manage. 63(2):517-523.

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: northern harrier. Northern Prairie Wildlife Research Center. Jamestown, ND. 12 pp.

MacWhirter, R.B. and K.L. Bildstein, 1996. Northern Harrier, Circus cyaneus. In A. Poole and F. Gill (eds.) The Birds of North America, No. 210. The Academy of Natural Sciences, Philadelphia, and The American Ornithologists' Union, Washington, DC.

Newton, I. 1979. Population ecology of raptors. Buteo Books, Vermillion, SD.

Serrentino, P. 1992. Northern Harrier. In Schneider and Pence (ed.) Migratory Nongame Birds of Management Concern in the Northeast. USFWS, R5, Hadley, MA:89-117.

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.

Toland, B.R. 1986. Nesting ecology of Northern Harriers in southwest Missouri. Missouri Academy of Science 20:49-57.

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