Wild Neighbors: Habitat for Harriers and the Restoration Paradox

By Joe Eaton
Wednesday March 11, 2009 - 07:37:00 PM
A female northern harrier cruising for voles.
Ned Kroeger
A female northern harrier cruising for voles.

Tinkering with the natural world often invites unintended consequences. Replacing exotic weeds with native vegetation is usually a laudable goal. But what if a sensitive species has its own plans for the weeds? 

That situation arose last month at the Berkeley Meadow, the parcel of land across University Avenue from the Seabreeze Market. It’s part of the East Shore State Park, managed jointly with the East Bay Regional Park District. For years the former landfill was covered with ruderal vegetation, plants like fennel, pampas grass, Himalayan blackberry. My dictionary defines “ruderal” as “growing in rubbish, poor land, or waste places.”  

The meadow has evidently been good enough habitat for California voles, small rodents that are a staple food for many birds of prey. The voles attracted short-eared owls, white-tailed kites, and the sensitive species in question, northern harriers. Northern harriers are listed by the California Department of Fish and Game as a Species of Special Concern, largely because of habitat loss. 

You may have noticed these long-winged hawks flying low over fields or marshlands, their wingtips tilted up in a dihedral. “A lazy, loafing, desultory flight it seems,” writes Arthur Cleveland Bent, “but really it is full of purpose, as it quarters low over the ground in a systematic search for its prey.” Males are pale gray above with striking black wingtips; females brown with white bellies; juveniles brown above, reddish-brown below. In all plumages, a white patch at the base of the tail is diagnostic. 

Harriers—about a dozen species worldwide, if you treat the North American northern harrier, Eurasian hen harrier, and South American cinereous harrier as separate forms—hunt by ear, as owls do. Their owl-like facial discs concentrate sound, enabling them to locate unseen voles rustling in the vegetation. Harriers have outperformed kestrels and other hawks in acoustic prey location trials. 

These adaptable birds have been seen tracking fencerows, ditches, or roadsides; hunting at the edge of fires, to catch prey rousted by the flames; scouting newly-mown hayfields for exposed nests; following foxes to pick up whatever they flush; even foraging in active bombing ranges. They’ll take meadowlarks and other songbirds, waterfowl up to the size of a teal or coot, and the occasional fish. Mostly, though, it’s voles. 

Most hawks nest in trees or on cliffs, but harriers (except for the spotted harrier of Australia) are ground nesters. Northern harrier nests have been found in marshes, grain fields, weed patches, cutover forest, young conifer plantations, and sagebrush. They’re more likely to be located near damp areas, where the voles are. Even in dry places, there will usually be a creek or stock pond nearby. The common requirement seems to be thick cover. Harrier nests are hard to find. 

Both sexes seem to be involved in selecting the site. A male harrier’s courtship sky-dance—a series of deep, U-shaped undulations—often ends with a plunge to the ground at a potential nest location. The male may start work on the nest; she’ll either finish it or construct her own nearby.  

The nest begins as a shallow platform on the ground, built up with sticks, straw, reeds, and weed stems and lined with feathers or moss. Wetland nests may be floating rafts. The male brings material, handing it off in an aerial pass or dropping it at the site. A pair may use the same nest several years in a row, refurbishing it annually. 

The northwest corner of the Meadow seems to have what harriers are looking for. Corinne Greenberg, who has been keeping an eye on the area for a long time, says they’ve nested there since the mid-90s. Although males are often polygamous, only one female at a time has been observed here. The vegetation may be weedy, but it’s dense enough; and the prey base supports both the harriers and a pair of white-tailed kites. 

Greenberg has also been keeping track of the Park District’s restoration work. The District has proceeded in three phases, starting with the area just north of University. Work on Phase 3, encompassing the northwest corner, began this winter, with crews clearing out the fennel and other weeds to be replaced with native plants. 

Last month she discovered that one of the two weedpatches where the harriers had previously nested had been taken out. She and others had seen the pair in the vicinity and even watched them mating, a likely sign of intent to nest. Greenberg contacted the Park District and the Department of Fish and Game, and put out an alert on the East Bay Birders listserve. 

That’s how Ron and I wound up at an informal stakeholders’ meeting at the Seabreeze Market. More on that next time.