Early November is a time of anticipation. The month brings winter to our area, of course, and creatures of all kinds adjust to new conditions, more or less appropriately. So it is with humans and so it is with birds, as well.

Early November thus brings uncertainty about which one of the northern bird species will show up first. The list of candidates is fairly short, and the usual winner is the rough-legged hawk.

True to expectations, the first rough-legged hawk of the season appeared in a solitary tree at the edge of an open grassy area along Grand Forks County Road 33 last week. This is my usual route from Magpie Ridge, the place west of Gilby, N.D., that I have shared with Suezette since the Flood of 1997 chased us out of the Riverside neighborhood in Grand Forks. In the 22 years since, the rough-legged hawk has become a kind of metronome, marking the shift toward winter.

The bird plays this role across the Northern Hemisphere. It is a circumpolar species whose nesting range includes some of the world’s rockiest, most desolate, windswept land and most northerly landscapes. There it subsists on rodents, including voles and lemmings. The population of rough-legged hawks fluctuates with the availability of prey, a factor pretty much unknown and underappreciated by most of us — until the birds appear.

Not surprisingly the rough-legged hawk is a bird of open spaces on migration just as it is during nesting season. The birds can be numerous in the swathe of grassland that remains along the first beach of glacial Lake Agassiz. This is an area of saline soils; often these are waterlogged. For generations, they have remained in a more or less natural state, used mostly for hay land and supporting a wide variety of grassland species.

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This is the habitat that sustains short-eared owls, for example. The owls had a successful nesting season, suggesting that prey species were abundant. Other open country nesters include northern harriers, also dependent on small mammals as prey.

The relationship between these predatory species is of great interest; how did two distinct species come to occupy the same habitat and exploit the same resources? Why is one species more successful in some years than the other, and why in some years — like this one — do both species seem to do well? What are the variables that influence prey populations, and what role do these predators play in the long-term populations? And how will predators and prey alike adapt to changes in the environment, including conversion of grassland to cropland through use of tile drainage? As the practice spreads, will the birds hang on?

Northern harriers and short-eared owls are largely migratory, though both species hang on here as long as the hunting remains good. Local numbers of both species decline, however, because food demand goes up as the temperature goes down, and the balance necessary is easier to maintain where temperatures are higher and animals more active.

The quality of light probably influences the movement of birds, as well; one thing that separates hawks and owls is the preference for hunting at specific times of day. Harriers are daylight hunters. Owls are crepuscular, hunting early and late in the day, although this season, owls seemed to be active well into the morning hours, potentially increasing the competitive pressure on the harriers.

Rough-legged hawks are daytime hunters. They are mostly perch-and-pounce predators. Individuals find an exposed perch — a tree, a utility pole, a fence post, an exposed rock — and keep watch for available food items. Some evidence suggests they are able to track the scent trails these animals leave. Rough-legged hawks also have the capacity to hover, unusual in such a large bird. These traits make the rough-legged hawks readily observable — and thus all the more anticipated.

A drive along any of the roads crossing the Grand Forks County grasslands — or similar areas either side of the Red River — could produce rough-legged hawk sightings. The birds are large, comparable to crows in body mass and wingspan and rivaling the size of red-tailed hawks, the most common perch and pounce predator in the summer months.

Rough-legged hawks appear rather mottled when they are at rest, dark overall, and often with a hunched posture. A rough-legged hawk on alert for prey looks large-legged; unlike other hawks, the legs are feathered to the ankles, an adaptation to life in cold places and the source of the birds’ name. In flight, rough-legged hawks are magnificent birds, displaying a distinctive white rump patch and a contrasting black and white pattern on the wings.

Jacobs is a retired publisher and editor of the Herald. Reach him at mjacobs@polarcomm.com.