BIOLOGICAL DATA AND HABITAT REQUIREMENTS
WILDLIFE SPECIES: Buteo lagopus | Rough-Legged Hawk 



TIMING OF MAJOR LIFE HISTORY EVENTS : 
Breeding:  Age at first breeding is 2 to 3 years [22].  Rough-legged
form pair bonds that are maintained for at least the duration of the
breeding season and possibly longer.  There is some evidence that pair
bonding occurs in wintering areas; birds roost and perch in twos and
sometimes migrate in twos [28].  After the loss of a mate, a new mate is
usually acquired fairly rapidly [22].  There is usually only one brood
per season [12].

Spring Migration:  Rough-legged hawks travel in loose flocks; up to 10
birds may be seen at a time, though hundreds might pass a hawk station
in a day.  Rough-legged hawks are not averse to crossing wide bodies of
water, which is unusual for a buteo.  They migrate across boreal forest
to find open country [28].  In the western part of rough-legged hawk
range, spring migration begins in late March or early April, with the
largest flights in late April.  Breeding pairs arrive on the breeding
grounds in late April to early May [37].  In the eastern part of its
range, rough-legged hawk migration occurs from early March to late April
or the first week of May; the peak period is in late March [20].

Nest:  The nest is constructed of sticks, bones, other debris, weeds,
and grass, and is lined with grass, down, feathers, and the fur of prey
animals [12].  Nests tend to be larger in areas where more sticks are
available.  Typical nests range in size from 24 to 30 inches (61-76 cm)
across and 20 to 22 inches (50-55 cm) deep [37].  Nests are used
repeatedly and become larger as new material is added [22].

Clutch:  Eggs have been observed in rough-legged hawk nests from May to
June, sometimes as late as July [34].  Earliest egg dates were May 2 in
Labrador and May 18 in arctic Canada and Alaska.  Latest egg dates were
June 23 in Labrador and July 13 in arctic Canada and Alaska [28].  In
captivity eggs are laid at 2-day intervals; on average, clutches of five
eggs take 10 days to lay [22].  Clutch size is variable and ranges from
two to seven eggs; clutches are smaller when prey is scarce [12].
Clutches of five to seven eggs are common in good lemming (Lemmus and
Dicrostonyx spp.) years; clutches of two or three eggs are more common
in poor lemming years [22].

Incubation:  Eggs are incubated for 28 to 31 days, mostly by the female.
The male feeds her, guards the nest, and incubates the eggs for only
short periods [28,37].  Eggs hatch asynchronously [12].

Development of Young:  Rough-legged hawk chicks are semialtricial; at
hatching they are immobile and downy with eyes open.  They are fed by
both parents.  Age of first flight is usually between 36 and 40 days
[12].  Most rough-legged hawk young leave the nest in early July to
mid-August at approximately 6 weeks of age but continue to depend on the
parents for food for a short period thereafter [37].  Dependence on the
parents for food sometimes extends to fall migration [28].

Fall Migration:  There is usually no large and distinct peak for
autumnal rough-legged hawk flights as there is in the spring [20].  At a
hawk-watching station on the shore of Lake Superior, peak flights of
migrating rough-legged hawks occurred from October 13 through the 31st.
The rough-legged hawk arrives in wintering areas in September and
October, and is settled from November through March [22].

Longevity:  The average life span of the rough-legged hawk is 20.7
months [28].  Rough-legged hawks have been reported as old as 6 years 9
months [34] and 18 years 1 month [28].

Population Fluctuations:  There are wide fluctuations in local winter
populations of rough-legged hawk in the southwestern United States [4].
Localized nonseasonal migrations of rough-legged hawk occur when prey
populations crash in usual breeding areas and the hawks move to areas of
more abundant prey [12].

Diurnal Patterns in Winter:  Most rough-legged hawks are active during
sunlight hours and retire to night perches by 4:30 p.m. [28].  Peak
activity is usually observed during periods of high wind velocity, clear
sky, rising air pressure, low relative humidity and high temperature [37].

Diurnal Patterns in Summer:  In Finland, rough-legged buzzards were
observed active by 4 a.m. in June and July.  They remained active
throughout the day until 8 p.m. (sometimes as late as 10 p.m.) [29].
Rough-legged hawks have occasionally been observed hunting between 12 a.m.
and 3 a.m. in arctic Alaska [37].


PREFERRED HABITAT : 
Nesting:  Rough-legged hawk nests are usually built on cliffs, river
bluffs, rocky outcroppings and ledges, columnar rocks, artificial
structures such as cairns, on the ground on steep hillsides, and rarely,
in trees [11,22,28].  Sites with an overhanging ledge or caprock are
preferred [22].  There appears to be a tendency to nest in clusters of
breeding pairs; however, this may be a function of nest site
availability [28]. Estimated average breeding density is approximately
one pair per 3.1 square miles (7.8 sq km).  Highest recorded density was
one pair per 1.6 square miles (4 sq km); however, there is often only
one pair per 31.2 square miles (78 sq km) [8].  Near the Colville River,
Alaska, nests averaged 2.3 linear miles (3.8 km) apart, ranging from
0.24 to 13.2 miles (0.4-22 km) apart.  Rough-legged hawks usually return
to the same nest site from year to year, even in the face of heavy
competition from peregrine falcons (Falco peregrinus) or gyrfalcons (F.
rusticolus) for the same site [22].  Rough-legged hawks are not as
aggressive as the late-migrating peregrine falcons, which often displace
rough-legged hawks from nest sites [22,28].

Hunting:  The rough-legged hawk prefers to hunt in open areas:  wet
meadows, bogs, marshes, riparian areas, pastures, and shrub-grass
uplands [4,24].  Hunting territory size during the breeding season is
variable; it may be as small as 2 to 2.4 square miles (5-6 sq km) when
prey density is high [22].

Wintering:  Rough-legged hawks usually winter in open country:
farmlands [32], plains, prairies, airports and other open urban areas,
coastal marshes, and agricultural lands.  The winter home range usually
ranges from 4 to 6 square miles (10-15 sq km) [22].  Competition where
winter ranges overlap those of the red-tailed hawk (Buteo jamaicensis)
is mitigated by behavioral differences in hunting styles; for example,
rough-legged hawks hunt from lower perches, prefer more open areas, and
avoid snow cover more than red-tailed hawks do [37].  The rough-legged
hawk is absent from northern regions where the average minimum January
temperature is less than -10 degrees Fahrenheit (-23 deg C).  This hawk
tends to avoid the western coastline and the southeastern corner of the
United States.  It is most abundant in areas with less than 40 inches
(1,020 mm) annual precipitation [28].


COVER REQUIREMENTS : 
Nest sites appear to be selected at least partly for a wide view [22,29]


FOOD HABITS : 
During the breeding season, the rough-legged hawk preys primarily on
microtine rodents (Microtus and Peromyscus spp.), brown lemming (Lemmus
sibericus), Nelson's collared lemming (Dicrostonyx groenlandicus),
tundra vole (M. oeconomus), singing vole (M. miurus), northern
red-backed vole (Cleithrionomys rutilis), and other small mammals [4].
Lemmings may comprise 80 to 85 percent of the summer diet [28].  In the
Northwest Territories brown lemmings comprised 83 percent of
rough-legged hawk summer diet, with lesser amounts of collared lemming
and arctic ground squirrel (Spermophilus parryi) [37], and
occassionally, Alaska hare (Lepus othus) [28].  Other food items include
insects and carrion [12].  Rough-legged hawks have been observed
consuming ringed seal (Phoca hispida) and caribou (Rangifer tarandus)
carcasses [39].  Springer reported that up to 30 percent of breeding
season diet in Alaska was avian prey, and consisted mostly of fledgling
passerines, ptarmigan (Lagopus spp.), and occasionally lesser
golden-plover (Pluvialis dominica) [33].  Palmer and Mindell [28]
reported that avian prey of rough-legged hawks in Alaska included grouse
(probably spruce grouse [Dendrapagus canadensis]) chicks, shorebirds,
lesser golden-plover, red-necked phalarope (Phalaropus lobatus),
whimbrel (Numenius phaeopus) chicks and juveniles, and other small birds
such as wagtail (Motacilla spp.), American tree sparrow (Spizella
arborea), Lapland longspur (Calcarius lapponicus), and snow bunting
(Plectrophenax nivalis) [28].

Winter diet is almost exclusively small mammals [4].  Palmer and Mindell
[28] estimated that voles comprise up to 80 or 90 percent of the winter
diet of rough-legged hawks, but occasionally birds as large as
ring-necked pheasant (Phasianus colchicus) and gray partridge (Perdix
perdix) may be taken [22].  On occasion weasels (Mustela spp.), ground
squirrels (Spermophilus spp.), rats (Rattus spp.), house mouse (Mus
musculus), and shrews (Crystotis, Sorex, and Blarina spp.) are eaten
[28].  In Iowa, wintering rough-legged hawks consumed meadow vole
(Microtus pennsylvanicus), western harvest mouse (Reithrodontomys
megalotis), and occasionally eastern cottontail (Silvilagus floridanus).
At another site in Iowa, deer mouse (Peromyscus maniculatus) and house
mouse were important in winter diets [37].  Wintering rough-legged hawks
have been observed fishing, and are also known to take frogs [28,37].  A
rough-legged hawk was observed stealing prey from a northern harrier
(Circus cyaneus), and rough-legged hawks were observed with other
raptors as prey, including sharp-shinned hawk [Accipiter striatus]) [28]
and short-eared owl (Asio flammeus) [27].

Hunting Style:  Much hunting is done from perches, including relatively
low sites such as fenceposts, poles, and even slightly elevated sites
such as rocks or mounds [22].  The rough-legged hawk frequently hovers
over one spot at an altitude of 50 to 132 feet (15-40 m) [28].  There is
also an appreciable amount of low-altitude flap and glide hunting for
mice; a rough-legged hawk often will quarter back and forth over open
fields [28,34].


PREDATORS : 
Rough-legged hawks have few natural enemies.  Terrestrial predators
include arctic fox (Alopex lagopus) and red fox (Vulpes vulpes), but
nests are usually so inaccessible and so well guarded as to preclude
much nest predation by foxes.  Gray wolves (Canis lupus) and golden eagles
(Aquila chrysaetos) have been observed eating young rough-legged hawks
in nests in Alaska [37].


MANAGEMENT CONSIDERATIONS : 
The rough-legged hawk is one of North America's most abundant raptors
[12].  Although no historical data on rough-legged hawk populations are
available, it is probable that the rough-legged hawk is currently stable
in North America [36].  White and Cade [38] assert that rough-legged
hawks are probably stable as a breeding population in Alaska, except
where oil and gas drilling sites, roads, pipeline development, and other
installations destroy nest habitat.  The estimated North American winter
population of rough-legged hawks was 49,600 based on 1986 Christmas bird
count data [12].  The maximum winter densities of rough-legged hawk
occurred in Montana and Idaho, with estimated state populations of 5,250
and 3,650 birds, respectively [22].  Several studies on the abundance of
rough-legged hawks are available [3,6,15].  Conservation and management
of the rough-legged hawk depends on factors affecting habitat in Canada
and the United States [7].

Population sizes and local abundance of rough-legged hawk are strongly
influenced by local prey populations [22].  Local rough-legged hawk
populations have been observed to increase and decrease with rodent prey
availability; local population size is apparently a function of hawk
movement to areas with abundant prey (rather than an absolute increase
or decrease).  For example, on the Seward Peninsula, there were 35
nesting pairs of rough-legged hawks in 1968, 43 pairs in 1969, and 82
pairs in 1970; however, only 10 pairs were found and two young fledged
in 1971 following a severe autumn in 1970 which held microtine
populations low.  In 1972, there were 44 nesting pairs [37].  In Norway
high density of breeding pairs of rough-legged buzzards corresponded
with high density of voles [19].  Palmer and Mindell [28], however,
asserted that fluctuations in vole populations probably influence but
are not the sole cause of rough-legged hawk population fluctuations
since rough-legged hawks shift to other prey when voles are scarce.  The
degree to which rough-legged hawk breeding success is independent of
vole population is related to the availability and use of alternate prey
[28].  Microtine rodent population fluctuations appear to be random
rather than cycling at 4-year or 10-year intervals, as has been
previously asserted, which further complicates understanding of the
relationship of rough-legged hawk populations to prey populations [18].
In Finland rough-legged buzzard nesting density was reported to be
independent of small mammal stocks; good nesting success occurred even
in poor vole years [29].  Winter rough-legged hawk populations are often
concentrated in areas of high prey density [37].

At a hawk migration station on Lake Superior, rough-legged hawks were
the most numerous raptor observed in 1979, even though 1979 was probably
a low population year for rough-legged hawks [13].  Migration counts of
rough-legged hawks at Bake Oven Knob, Pennsylvania, were variable,
increasing and decreasing in what appeared to be 3- to 5-year cycles.
Cycles were also reported for migration counts at Hawk Mountain, Maine
[21].  In New Jersey wintering population densities of rough-legged
hawks varied widely between years but showed no obvious upward or
downward trend [7].

Rough-legged hawks banded in California bred in several locations, from
Alaska to Banks Island in the Northwest Territories.  Encounters with
banded rough-legged hawks suggest that rough-legged hawks from the
western part of the breeding range migrate to the western parts of the
winter range.  Rough-legged hawks also show strong fidelity to the same
winter area in subsequent years; this suggests that the loss of
wintering habitat to residential development could be detrimental to
rough-legged hawk populations [16].  Wintering concentrations of
rough-legged hawks in the Great Plains and the Intermountain West were
correlated primarily with climate and the presence of protected areas
such as wildlife refuges.

The rough-legged hawk is most common in the Great Basin and the Northern
Great Plains.  Winter populations are high in Montana, northern Oregon,
southern Washington, and northeastern Maine north to Newfoundland [30].
A drive-by survey in southeastern Idaho revealed that rough-legged hawks
were the most abundant raptor, appearing most often on agricultural land
[10].  Wintering rough-legged hawks are widely distributed throughout
New Jersey, but appear to prefer coastal areas.  Abundance in New Jersey
was significantly correlated (p < 0.002) with wetlands; rough-legged
hawks also appear to avoid areas with snow cover [7].

Mortality Factors:  The rough-legged hawk is often relatively
unsuspicious of human approach.  Prior to their legal protection,
rough-legged hawks were shot in large numbers in wintering areas of the
United States or were caught in pole traps.  In Utah large numbers of
migrant rough-legged hawks were reported killed by cars while feeding on
road-killed jackrabbits (Lepus spp.) [35,37].  In 1967 mortality from
dieldrin poisoning was documented in rough-legged buzzards in Britain.
Consumption of only a few animals that had eaten dieldrin-treated grain
was sufficient to kill the hawks [37].  


REFERENCES : 
NO-ENTRY

Related categories for Wildlife Species: Buteo lagopus | Rough-Legged Hawk