From the Archives
This article originally appeared in the Travis Audubon Newsletter in 2005. Photos by Jim deVries.
On a cold, cloudy January afternoon, the open prairie seems lifeless. To the human eye, there is at first no discernible movement, except for the dry grasses rustling in the brisk north wind.
Yet there, low over the grass, flies a long-winged hawk. The way it skims back and forth, it sometimes resembles a huge, lumbering swallow. Or, teetering unsteadily from side to side, the tips of its wings held higher than its body, it may look like a pale Turkey Vulture struggling to leave the ground. It hovers momentarily in mid-air, kiting into the wind, then drops to the ground. The prairie is not so lifeless after all, and an unlucky rodent has just become a meal for a Northern Harrier.
Harriers, of which the Northern Harrier is the only North American representative, are long-winged, long-tailed, long-legged hawks of open country. They are unusual among the hawks for having somewhat flattened faces, and facial discs like those of the owls.
Northern Harriers hunt grasslands and marshlands across the northern hemisphere. In North America their preference for nesting in wet meadows once earned them the name Marsh Hawk. The species is called Hen Harrier in England, where the birds often occupy drier upland sites as a result of competition with two other harrier species. When they come to central Texas in the winter, Northern Harriers search for prey in both wet and dry grasslands.
The flight style of Northern Harriers seems to be what human observers notice most about them. William A. Quayle in The Prairie and the Sea apparently thought it erratic: “[Harrier] flight has the notionateness of prairie winds, and the sudden detour as a change of mind, a leap straight on, and then a notionate, abrupt change in direction as if he had just bought wings and were trying what sort of wings they were.” Henry David Thoreau was more impressed: “I look up and see a male marsh hawk with his clean-cut wings, that has just skimmed past above my head,– not at all disturbed, only tilting his body a little, now twenty rods off, with demi-semi-quaver of his wings. He is a very neat flyer.”
If its shape and behavior don’t immediately identify a harrier, the flash of a small white patch above its tail will. Both sexes of all ages wear the white patch on the upper tail coverts (sometimes called the rump, which is actually a little higher up the back). Otherwise, adult males and females have quite different plumages, a phenomenon called sexual dichromatism, which, though common among passerine birds, is rare among our hawks.
The males (which tend to winter farther north and so are less common here) are ghostly gray with a white belly and black wingtips. The females are brown above, with extensive brown streaking on whitish breast and belly. First-year birds resemble adult females, but have cinnamon to buffy underparts, with much less streaking that is confined to the flanks and upper breast. Like other hawks, the males are noticeably smaller than the females.
Mice are so important to the Northern Harrier’s diet that one researcher, Frances Hamerstrom, has called it “the hawk that is ruled by a mouse.” Her studies on their breeding grounds showed that harriers feed primarily upon voles of the Microtus genus, but other studies have concluded that they are much more opportunistic, shifting to fledgling birds when those are plentiful. Given that voles are scarce in central Texas, the harriers that winter here must rely upon other prey. Cotton Rats, maybe? There are certainly plenty of those in the harriers’ habitat hereabouts.
Several other predators – such as Gray Foxes, Western Diamondback Rattlesnakes, and Red-tailed Hawks – prey on rodents, too. All of these predators may be hunting the same field at the same time, which could present a problem of competition. Fortunately (for the predators, not for the rats), they have evolved different hunting abilities. Foxes have a keen sense of smell. Rattlesnakes can detect the body heat of their prey. The hawks have acute eyesight, and the ability to fly.
The two hawks have evolved different hunting strategies. Red-tailed Hawks like to watch from on high – either soaring beyond a rodent’s visual range or sitting still in a tall tree (or on a utility pole). Harriers take the low road, barely clearing the tops of grasses as they course back and forth over their hunting grounds. In a way, they fly under a mouse’s field of view; by the time a mouse sees a harrier coming there’s too little time to get away. Harriers are also adept at using obstacles as cover – sweeping quickly around a grove of trees or cutting crosswise over a grassy ditch – to catch their prey by surprise.
The harriers’ facial structure also helps their hunting. The owl-like facial disk funnels sound to the ears, enhancing a harrier’s hearing, so a harrier can hunt by sound, like the owls, as well as by sight, like other hawks. In fact, a Northern Harrier can capture prey by sound alone from a distance of 3 to 4 meters – which is another reason for flying so low. A scurrying mouse hidden from the Red-tailed Hawk’s keen eyes can thus be detected by the Northern Harrier’s hearing.
The Northern Harrier’s “notionate” flight style is part of a successful hunting strategy, though it may look indecisive to us. Then again, sometimes they show us just what mastery of flight means:
“Suddenly, over the slope, as if tethered to a cord of air drawing quickly upward, came a northern harrier, motionless but for its rising. So still was the bird – wings, tail, head – it might have been a museum specimen. Then, as if atop the wind, it slid down the ridge, tilted a few times, veered, tacked up the hill, its wings hardly shifting. I thought, if I could be that hawk for one hour I’d never again be just a man.” (William Least Heat Moon, PrairyErth)