©Catherine Hamilton, all rights reserved.
Catherine Hamilton is an internationally recognized artist and natural history illustrator. A former instructor at the Rhode Island School of Design, she has exhibited her paintings and drawings over the last 25 years at galleries and small museums across the United States, and has work in private and corporate collections in the United States and Europe.
Catherine was featured in the 2012 HBO documentary “Birders: the Central Park Effect.” She has developed a body of work that crosses the boundaries between artistic and scientific investigation. In addition to gallery exhibition, her work recently has been published in the scientific journal Nature and in The Warbler Guide (Princeton University Press 2013).
Northern Hawk Owl
The Northern Hawk Owl (Surnia ulula) is well named, for in shape and behavior this diurnal owl resembles hawks of the genus Accipiter. A hawk owl will often perch conspicuously in a treetop and in flight will often glide low or flap its somewhat pointed wings with quick, stiff wingbeats. It has a long tail for an owl. Its back and wings are patterned brown and white, it is barred with rufous below, and its grayish face is edged in black. It is similar in size to the Long-eared Owl. The Northern Hawk Owl’s size, long tail, and perching and flying behavior separate it from other owl species. Much of what we know about the behavior of the Northern Hawk Owl comes from studies of Eurasian birds, as it is one of North America’s least studied bird species.
The Northern Hawk Owl is a denizen of the circumpolar boreal forest where three subspecies are recognized; only S. u. caparoch breeds in North America. Its breeding range extends from Alaska in a broad swath across Canada to Newfoundland and south to the Great Lakes. Hawk owls are year-round residents of this area but are nomadic and may periodically wander hundreds of miles in search of high concentrations of prey. They are occasionally irruptive, moving south into the northern United States when harsh winter conditions and scarcity of prey prevail, especially following successful breeding seasons. In Massachusetts, Northern Hawk Owls are vagrants with only about a half dozen records since 1900. One bird, present in Concord from November 1958 to January 1959, was the last life bird of the legendary birder, Ludlow Griscom.
The Northern Hawk Owl is a usually monogamous, single-brooded species. Both males and females have advertising calls. One of the male’s call is described as a trilling whistle: ulululululululul given either from a perch or during a circular display flight involving gliding on stiff wings. The female may duet with the male, sometimes bowing or actually touching heads with her mate. Alarm calls have been described as rike, rike, rike or kip-kip-kip, or as a screech including various trills.
Northern Hawk Owls may begin nesting as early as February in existing tree cavities. They may line the nest with feathers and fur. The clutch size is variable but averages about six white eggs. Only the female has a brood patch and she alone broods the eggs for about four weeks until hatching. The male brings her food while she is incubating. Both parents will attack an intruder, including humans. At hatching, the altricial young have closed eyes and are naked and helpless. Only the female broods the chicks for three to five weeks until fledging. During this period the male brings food to the female and she feeds the chicks. The young are not fully independent until they are about three months old; they remain in the nest area for several months.
The habitat of Northern Hawk Owls is open coniferous forests or mixed coniferous and deciduous forests that are near open areas such as marshes, bogs, or, in winter, agricultural fields. Hawk owls often hunt in open habitat from exposed perches in nearby trees. They probably detect prey more by sight than hearing, because they lack the asymmetrical ear holes of owls that rely heavily on hearing to track prey. Hawk owls descend on unlucky voles or squirrels in gliding dives that are interspersed with flapping if the distance is great. They may hover or occasionally employ searching flights in fields at heights of up to twelve feet above the ground. They have been recorded using farm equipment as “beaters,” following and taking prey disturbed by the vehicles. Their diet in summer is mostly small mammals including voles, squirrels, hares, and lemmings. In winter they may take mostly birds, including grouse, ptarmigan, and songbirds. They typically swallow small mammals whole, or else swallow the head first and then eat the body, sometimes after eviscerating it.
Great Horned Owls and Gyrfalcons prey upon Northern Hawk Owls, and mammals of the weasel family are serious nest predators. Humans often shoot hawk owls because they are mainly diurnal, absurdly tame, and habitually sit on exposed perches. They have even been labeled “practice owls” by some. Because of the remoteness of their breeding habitat and their nomadic behavior, breeding bird data is inconclusive in assessing population numbers and changes. Apparently, populations fluctuate in response to periodic crashes in preferred prey, but because of their dispersive nature it is difficult to quantify changes. The Northern Hawk Owl is currently considered “Not at Risk” in Canada and is “Globally Secure,” which bodes well for the survival of this elegant owl species.
William E. Davis, Jr.
Northern Hawk Owl study ©Catherine Hamilton, all rights reserved.