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Northern Harrier

 

By Lee Franks

Male Northern Harrier

Male Northern Harrier © Bob Steele

The Northern Harrier (aka marsh hawk) is a slender, white-rumped, medium-sized, and low flying raptor with an owl-like face. It can best be identified by its flight behavior, tracing wavy lines over grasslands and marshes, often retracing its path several times in the quest for prey. It employs slow, lazy wing beats which coincide with its undulating, erratic flight pattern as this raptor skims the tall grassy meadows on the west side of the Park with its belly. Unlike other raptors, which can find their prey only visually, the Northern Harrier stays close enough to the ground to listen for birds (nesting or fledgling song birds), voles, and mice. When stirrings catch its eyes or ears, it abandons its lazy ways to strike at prey with astonishing energy.

Harriers are year-round residents throughout California, but they appear to be more abundant during fall and winter months around here because the residents are joined by individuals from the northern-most areas of their range (Alaska and Canada) who migrate as far south as Central and South America. It is likely that the one or two that we see hunting in the Park during the winter months are migrants that choose to stop over on their journey south, or stay in the Park just for the winter.

Appearance

Female Northern Harrier

Female Northern Harrier © Garth McElroy

There is a big difference in appearance between the male and the female. Adult females are about 50% heavier and 12% larger than adult males. The female is brown above, whitish below, with heavy brown streaking on the breast and flanks, and lighter streaking and spotting on the belly. The facial feathers are brown, but strongly outlined with white cheek feathers. The male is gray above and mostly white below, with bold black wing tips. Their tail is darkish gray above and whitish below. Males lack the distinctive facial feathers of the female.

Both sexes have a white rump patch, owlish face, and eye color that changes from brown to yellow as they age over a three year period. The evolutionary significance of this eye color change is unclear, but some feel that it may serve to help determine the maturity of potential mates.

Behavior

Harriers forage over open habitats. The frequency of use of certain habitats appears to be related to a combination of prey biomass and vegetative cover. Areas of short vegetation are under-used, whereas idle and abandoned fields with vegetative cover are used frequently. Males seem to prefer more open habitats than females. This difference is no doubt related to 1) use of different prey species (males take more birds), 2) the smaller home range of females relative to those of males, which results in female preference for habitats surrounding nest sites, and 3) female exclusions of males from preferred hunting habitats during winter months. Females hunt in taller and denser vegetation than do males. When we see this raptor during our monthly surveys in the Preserve, it is either male or female, flying or perched, solo. Seldom, if ever, do we see the two sexes together.

Male Northern Harrier in flight

Male Northern Harrier © Bob Steele

Most prey pursuits are short temporally and spatially, and close to the ground. They sometimes use the cover of vegetation and terrain to surprise prey. Prey-capture success is highly variable (5-35% of pounces are successful), depending on habitat, prey type, and age or sex of the individual harrier. When captured, large prey items, especially birds, are plucked and eaten, usually on the ground, but sometimes on elevated perches. Smaller items are swallowed whole.

In general, males fly faster than females or juveniles, regardless of the type of flight. Males have shorter wings and lower wing-loading than females do, and appear more agile in flight. In the fledgling stage, juveniles chase and supplant one another, and occasionally pounce and play with inanimate objects.

Breeding

Pair formation occurs on their breeding grounds. Adult males generally arrive 5-10 days before females. Aerial courtship and territorial displays coincide with the arrival of the adult female. Male or female may select the nest site. Often the male will initiate nest building by creating platforms in the presence of the female, stimulating her to complete the nest. Nests are built on the ground within patches of dense vegetation. Both sexes carry nest material to the nest site, although most platforms are added to and lined by the female. Males transfer material to the female by an aerial pass or on the ground at the nest site. Construction requires 7-14 days, using grasses, forbs, weeds, rushes, etc.

Eggs, which are spotless, smooth with little gloss, are laid at 2-3 day intervals. Clutch size is 5-6 eggs, and normally one clutch per season is laid. The female alone incubates the eggs. Eggs hatch in the sequence in which they were laid, on average at 2-day intervals. Young are covered with short, white down at hatching. Their eyes open within hours of hatching, and they are able to crawl to the edge of their nest to defecate, stretch, and receive food. The female only broods, beginning immediately after hatching. Diurnal brooding ends when the oldest nestling is about 12-14 days old, but the female will brood older young during rainy weather. Nocturnal brooding continues until 28-30 days, shortly before offspring begin flying.

Although the center of activity may move several hundred meters from the nest, fledglings spend most of the day waiting on elevated perches for their parents to return with food. Usually only about 20% of their day is spent flying. Most flights appear to be exercise flights in which the birds fly in wide circles before returning to their original perch sites. Once all siblings are capable of flight, virtually all food items are exchanged by an aerial pass, and prey usually are relinquished to the first fledgling that reaches the parent. Ability to secure food from parents is strongly influenced by the sequence in which siblings begin flying. Fledglings practice prey capture by pouncing on inanimate objects, but spend little time, if any, hunting, and rarely capture prey prior to independence.

Mortality rates are estimated at 59% in first year and 30% among adults. The longest lifespan reported is 16 years, 5months.

References

The Birds of North America, No 210, 1996; R. Bruce Macwhirter and Keith L. Bildstein.

This article was originally published in the Edgewood Explorer, March 2007.

Learn More…

Northern Harrier at All About Birds. This guide includes a number of Northern Harrier sounds.

Northern Harrier in the Audubon Field Guide