Northern Harrier: the Hawk Ruled by a Mouse
Northern Harrier by Chuck Musitano
CURRENT STATUS: In Pennsylvania, Northern Harriers (Circus cyaneus) are categorized by the Pennsylvania Biological Survey as a Candidate – At Risk because of its risk of becoming Threatened if losses continue. And it is categorized by the State Wildlife Plan as a ‘high level of concern’ because of its small, localized and vulnerable populations that are scattered in pockets of open wetland, grassland or farmland habitat. They are protected under the Game and Wildlife Code. Nationally, harriers are protected under the Migratory Bird Treaty Act of 1918.
POPULATION TREND: Northern Harriers were rare, but regular breeders in the state through the mid-1900s, primarily near rivers and lakes in the northern and northwestern counties; however a few nests were found at the Philadelphia Airport until the 1980s. Decline in the North American population of Northern Harriers, attributed to habitat loss, were noted in the 1960s through the 1980s. Wetlands were drained for agriculture and suburban development and old fields reforested or were converted for other uses. Since then, populations have stabilized or declined at slower rates. Results from the 2nd Pennsylvania Breeding Bird Atlas (2004-2009) indicate that Northern Harriers are still rare breeders in the state, however. Significantly fewer birds were observed during this recent atlas effort than during the 1st Pennsylvania Breeding Bird Atlas in the late 1980s (1985-1989). There have been declines in the migrating population noticed at Hawk Mountain Sanctuary in recent decades but other hawk watch sites have had either non-significant changes or increases. Migration seems very tied to fluctuations in its small rodent prey base. Harriers do indeed seem to be "ruled by a mouse" in all seasons.
IDENTIFYING CHARACTERISTICS: Northern Harriers are often observed in flight, hunting low over open fields of tall grasses, or perched on fence posts. They can be distinguished from other open country raptors, such as Rough-legged (Buteo lagopus) and Red-tailed Hawks (Buteo jamaicensis), by their narrow wings forming a V-shape in flight, long tail, dark wing tips, and white rump patch. Rough-legged hawks, arctic breeders and winter visitors to the state, and Northern Harriers both hover briefly over prey items. Observing the white rump patch of Northern Harriers will lead to a positive identification. Rough-legged Hawks are generally larger birds with broader wings, dark wrists, and a white band at the base of the tail. Short-eared Owls (Asio flammeus) hunt the same fields but in the evening and at night rather than in the day. Northern Harriers are unique among our birds of prey because sexes can be determined by plumage differences. Males are grey above and light below, while the larger females are brown above with buff and streaks of brown on her chest. The face of Northern Harriers bears more resemblance to that of an owl’s than a hawk’s. Most hawks are visual predators, however facial disks around the harrier’s eyes direct sound to their ears, allowing them to hunt by sound as well as sight.
BIOLOGY-NATURAL HISTORY: Formerly known as the ‘marsh hawk’, Northern Harriers are found in open wetlands, meadows, grasslands, farmland, thickets, and riparian woodlands. They winter across the middle to southern United States into Central and South America and the Caribbean Islands. Some birds travel over 900 miles to their wintering grounds. Northern Harriers are rare but regular winter residents in Pennsylvania and may be spotted hunting over large, open fields. Communal ground roosts also have been encountered. Nesting begins in late April – mid May. The breeding ecology of the species is strongly tied to abundance of prey items, primarily mice and voles. Clutch size and nest density can fluctuate between years, with larger clutches and more nests per area occurring in years of high prey abundance. Nests are built on the ground of grasses and small twigs. Average clutch size is 4-6 eggs. When food resources are plentiful, males sometimes mate with more than one female. Females independently incubate the eggs for approximately 30 days and raise the young to fledging (4-6 weeks after hatching) while the male delivers food. They will defend nests from curious human visitors.
PREFERRED HABITAT: Northern Harriers prefer large, open wetlands, pastures, cropland, grasslands, bogs, thickets, and riparian woodlands. Reclaimed strip mines planted with tall grasses are also frequently used. Many of the 2nd Pennsylvania Breeding Bird Atlas observations were from grass fields planted through the USDA Conservation Reserve Enhancement Program.
REASONS FOR CONCERN: Pennsylvania has lost more than half of the wetlands that were here prior to European settlement. Although the rate of wetlands loss has significantly diminished over the decades due to legislative protection, these habitats are still threatened by development, pollution, sedimentation and fragmentation. As wetlands disappear, so do the species – like Northern Harriers - that inhabit those ecosystems. A growing concern to currently protected wetlands (e.g. Game Lands, Audubon Important Bird Areas, Land Trusts) is encroachment of non-native, invasive plants such as purple loosestrife (Lythrum salicaria). Invasive plant species out-compete native plants, therefore reducing biological diversity and limiting resources for wildlife in those wetlands. Intense management practices on farmlands also limit the ability of ground-nesting species such as Northern Harrier to nest successfully even if a good prey base exists.
MANAGEMENT PROGRAMS: The state Wildlife Action Plan identifies several management objectives to assist wetland-associated species. The paramount objective that would benefit Northern Harriers is to preserve, protect, and restore large (> 5 hectare), shallow wetlands with dense emergent vegetation. Many of the large wetlands where Northern Harriers have nested are preserved as Audubon Important Bird Areas (e.g. Conneaut and Geneva Marshes, Crawford County and Long Pond Preserve, Monroe/Carbon Counties). These, and other, wetlands must be protected from sedimentation, polluted runoff, and degradation by invasive species. By delaying harvest of grains and hay and avoiding mowing until late summer, the success of grassland birds such as Northern Harriers can be enhanced. Managing grassy and thicket habitat on a larger scale and in conjunction with wetlands nearby also would increase the chance for harriers to expand their range in the state and successfully nest in adequate numbers to maintain a viable population. Other species of conservation concern also benefit from a matrix of grassland, thicket, and wetlands. Local partnerships may be necessary for this kind of approach to wildlife habitat management. Also, knowing where these rare birds overwinter and breed will help biologists understand what sites are critical to the species’ survival. Pennsylvania eBird, a citizen-science project that allows birders to maintain a geo-referenced electronic bird list, will lend insight into these important areas.
Some Locations Good to Search for Northern Harriers
and Conduct eBird Surveys
Locations Better in Winter or Migration Are Marked by a (W):
Any large field in the Conservation Reserve Enhancement Program
Reclaimed strip mines grown back in grass
Bogs and open wetlands especially in northern counties
Establish a Hotspot at any site where you go birding for grassland and wetland birds.
Specific locations, most listed as Pennsylvania Important Bird Areas
Presque Isle State Park
Pymatuning Swamp and Lake
Conneaut Marsh / Geneva Marsh, State Game Lands 213
Erie National Wildlife Refuge
Mount Zion / Piney Tract
Pennsy, Black, and Celery Swamps, State Game Lands 284
Marsh Creek Wetlands (the “Muck”), State Game Lands 313
Long Pond Preserve and Barrens
Allegheny Front, especially its reclaimed strip mines
Sproul State Forest, especially burned over area
Southern Adams County Grasslands (W)
Middle Creek Wildlife Management Area (W)
Blue Marsh Lake (W)
Laurels, King Ranch, and Stroud in Chester Co. (W)
John Heinz National Wildlife Refuge (W)
Thanks to Chuck Musitano for use of his photographs of Northern Harrier flying in Columbia County, Pennsylvania.
Information Sources
Farmer, C. J., R. J. Bell, B. Drolet, L. J. Goodrich, E. Greenstone, D. Grove, D. J. T. Hussell, D. Mizrahi, F. J. Nicoletti, and J Sodergren. 2008. Trends in Autumn Counts of Migratory Raptors in Northeastern North America, 1974-2004. Pages 179 – 215 In State of North America’s Birds of Prey, Series in Ornithology No. 3 (K. L. Bildstein, J. P. Smith, E. R. Inzunz, and R. R. Veit, editors). Nuttall Ornithological Club, and the American Ornithologists’ Union, Printed by Cadmus Communications, Lancaster, PA.
Goodrich, L. 1992. Northern Harrier. In Atlas of Breeding Birds in Pennsylvania, D. Brauning, editor. University of Pittsburgh Press, Pittsburgh, Pennsylvania.
Macwhirter, R. Bruce and Keith L. Bildstein. 1996. Northern Harrier (Circus cyaneus), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology.
McWilliams, G. M. and D. W. Brauning. 2000. The Birds of Pennsylvania. Cornell University Press, Ithaca, New York.
Pennsylvania Game Commission and Pennsylvania Fish and Boat Commission. 2005. State Wildlife Action Plan. Harrisburg, Pennsylvania.
Suggested further reading
Broun, M. 2000. Hawks aloft: The story of Hawk Mountain. Stackpole Books, Pennsylvania. 222 pp.
Clark, W. S. and B. K. Wheeler. 2001. A Field Guide to Hawks of North America. 2nd Ed. Houghton Mifflin Co., Boston and New York.
Gross, D. A. and C. D. Haffner. 2010. Wetland bird communities: Boreal bogs to open water. In Avian Ecology and Conservation: A Pennsylvania focus with national implications. The Pennsylvania Academy of Science, Easton, Pennsylvania. 368 pp.
Hamerstrom, F. 1986. Harrier, hawk of the marsh: the hawk that is ruled by a mouse. Smithsonian Institute Press, Washington D. C.
Johnsgard, P. A. Hawks, Eagles, and Falcons of North America. 1990. Smithsonian Institution Press, Washington D. C.
Sibley, D. A., C. Sutton, P. Dunne. 1989. Hawks in Flight. Houghton Mifflin Harcourt, New York. 272 pp.
Weidensahl, S. 1996. Raptors: the Birds of Prey. Lyons and Burford, New York, NY.
Zalles, J. I. and K. L. Bildstein, Eds. 2000. Raptor Watch: A global directory of raptor migration sites. Birdlife International, Cambridge, UK, and Hawk Mountain Sanctuary, Kempton, PA, USA (Birdlife Conservation Series No. 9).
