BIOLOGICAL DATA AND HABITAT REQUIREMENTS
WILDLIFE SPECIES: Buteo swainsoni | Swainson's Hawk 



TIMING OF MAJOR LIFE HISTORY EVENTS : 
Age at sexual maturity - Swainson's hawk are generally sexually mature
at 2 years of age [31].

Nesting season - The Swainson's hawk arrives on its breeding grounds
later than most raptors [31].  The nesting season generally occurs from
March to October depending on geographic area [14,21,35].  In
California, the Swainson's hawk breeds from late March to mid-August,
with peak activity from late May to late July.  In Nevada, it breeds
from April to October [21].  In Montana, the breeding season is from May
to September [14].
 
Clutch size and incubation - The Swainson's hawk lays two to four eggs,
with two most common [14,21,31,35].  The eggs are incubated for 28 to 35
days [14,21,31].  The Swainson's hawk may lay a replacement clutch if
the first clutch is destroyed [31].

Fledging - Nestlings fledge in 35 to 44 days [21,31].  Fledglings
continue to be fed by the adults and remain within the nesting territory
for 14 to 21 days after fledging; they often return to the nest tree to
roost [21].

Migration - The Swainson's hawk travels in large flocks (sometimes
containing over 100 individuals) from the nesting areas south to their
winter grounds in South America [6,21,31].

Peak fall migration clears the southern plains states and southern Texas
by early October.  The Swainson's hawk arrives in Central America the
last 3 weeks of October to early November; arrival in Argentina is
reported as late November.  Average dates for spring migration of the
Swainson's hawk are mid-March in Panama, the last 3 week of March in
Costa Rica, the last half of March and first week of April in the state
of Veracruz, Mexico, and early April in southern Texas [31].

Longevity - The Swainson's hawk probably seldom lives longer than 16
years [31].


PREFERRED HABITAT : 
The Swainson's hawk inhabits mostly semiopen to open areas in tundra,
valleys, plains, dry meadows, foothills, and level uplands at low to
middle elevations [1,31,40].

Nesting habitat - The Swainson's hawk nests almost exclusively in trees
[37] and will nest in almost any tree species of suitable size (taller
than 10 feet [3 m] with a d.b.h. of 2 inches [5 cm] or more) [6,7].
Nests are constructed in isolated trees (dead or live), in trees in
wetlands and along drainages, or in windbreaks in fields and around
farmsteads [6,12,31].  The Swainson's hawk builds nests from 4 to 100
feet (1.2-30.4 m) above the ground [12,14,35].  They sometimes add to an
existing black-billed magpie (Pica pica) nest [31].  The Swainson's hawk
occasionally nests in shrubs, on the crossbars of telephone poles, or on
the ground, low cliffs, rocky pinnacles, or cutbanks [6,12,31].

In the Central Valley of California, the majority of Swainson's hawk
nests and territories are located in or near riparian systems.  Nests
are found most often in cottonwoods and oaks [33].  In Whitman County,
Washington, Swainson's hawk nests were constructed in black locust
(Robinia pseudoacacia), cherry (Prunus spp.) and hawthorn (Crataegus
spp.) trees [5].  Of 48 Swainson's hawk nests on the Laramie Plains,
Wyoming, 43 were in narrowleaf cottonwood (Populus angustifolia),
peachleaf willow (Salix amygdaloides), or other willows.  None of the
few buttes in the area were used for nesting [15].  In the Centennial
Valley of Montana, Swainson's hawks nest extensively in willows [32].
At 234 Swainson's hawk nest sites in North Dakota, eastern cottonwood
(Populus deltoides) was the most common tree species used (45%).  Other
species included Chinese elm (Ulmus parvifolia), 22 percent; peachleaf
willow, 13 percent; boxelder (Acer negundo), 12 percent; and green ash
(Fraxinus pennsylvanica), 7 percent.  Amercian elm (U. americana) and
Russian olive (Elaeagnus angustifolia) accounted for the remaining 1
percent [20].  In the Lower Sonoran Desert of New Mexico, the Swainson's
hawk often nests and roosts on large yucca plants [30].  In the
southwestern United States, mesquite is commonly used [6].

Foraging habitat - The Swainson's hawk generally forages in open
habitats with short vegetation containing small mammals, reptiles,
birds, and insects [6,40,38].  During the nesting season the Swainson's
hawk usually forages within 1.9 miles (3 km) of the nest.  The
Swainson's hawk has a home range of approximately 3.5 square miles (9 sq
km) [40].  Although the Swainson's hawk does search for prey from
elevated perches, it relies much more on aerial foraging.  Consequently,
it is not tied to habitats containing an abundance of perches, and often
occupy habitats with few or no perches except the nest tree [23].

Winter habitat - The Swainson's hawk generally spends the winter south
of the United States [1,12,31]; no information is available in the
English literature on its habitat in Central and South America.


COVER REQUIREMENTS : 
Swainson's hawk nests are often built in trees that provide shade for
the nest but also afford a good view of the surrounding terrain [33].

The Swainson's hawk is a more efficient predator in open areas than in
areas with high vegetative cover [5].  Bechard [5] found that vegetative
cover is more important than prey abundance in the selection of hunting
sites by the Swainson's hawk.  In Whitman County, Washington, the
Swainson's hawk foraged at sites where vegetative height and density had
been reduced, even though other areas had higher prey density [5].
Alfalfa field use by Swainson's hawk in northern California increased
dramatically during monthly harvests that reduced vegetative heights
[38].


FOOD HABITS : 
The Swainson's hawk is a versatile and opportunistic predator on
relatively small prey [6].  The Swainson's hawk feeds on small mammals,
large insects, birds, and reptiles [9,14,31,35].  During the breeding
season, the Swainson's hawk primarily preys on small mammals, especially
young ground squirrels (Spermophilus spp.), pocket gophers (Thomomys
spp.), and some microtines [15,20,31,32].  During migration
invertebrates often make up over 90 percent of the Swainson's hawk's
diet [31].

In a North Dakota study, Swainson's hawks preyed primarily on northern
pocket gopher (Thomomys talpoides), Richardson's ground squirrel
(Spermophilus richardsonii), meadow vole (Microtus pennsylvanicus), and
thirteen-lined ground squirrel (S. tridecemlineatus) [20].  To a lesser
extent Swainson's hawks also ate western meadow lark (Sturnella
neglecta), chestnut-collared longspur (Calcarius ornatus), sharp-tailed
grouse (Tympanuchus phasianellus), short-eared owl (Asio flammeus),
American kestrel (Falco sparverius), blue jay (Cyanocitta cristata),
and rock dove (Columba livia) [20].  Toads (Bufo spp.) and various
lizards, mostly desert grassland whiptail (Cnemidophorous uniparens)
and spiny lizards (Sceloporus spp.), were commonly taken by nesting
Swainson's hawk in Arizona.  Mammals, particularly cottontails
(Sylvilagus spp.), ground squirrels, and kangaroo rats (Dipodomys spp.),
were the most common food items brought to Swainson's hawk nestlings in
New Mexico [6].  Rabbits comprised between 40 and 80 percent of the diet
of Swainson's hawk nestlings in New Mexico [7].


PREDATORS : 
Large raptors including great horned owl (Buteo virginianus) and golden
eagle (Aquila chrysaetos), and coyote (Canis latrans), bobcat (Lynx
rufus), and striped skunk (Mephitis mephitis) have been known to kill
Swainson's hawk nestlings and fledglings or destroy clutches
[7,11,15,31,38].  Crows (Corvus spp.) sometimes destroy clutches
[15,31].


MANAGEMENT CONSIDERATIONS : 
Swainson's hawk populations have declined over much of their breeding
range due to habitat loss from cultivation, removal of riparian areas,
and removal of shelterbelts [14,31].  Nest site availability may limit
occurrence and breeding density of Swainson's hawk [6].  Schmutz and
others [34] reported that the nesting density of the Swainson's hawk
increased significantly following the erection of 98 artifical nest
platforms in an experimental study plot.  When roads, pipelines, or
other surface facilities are constructed, trees taller than 10 feet (3
m) with a d.b.h. of 2 inches (5 cm) or more should be conserved.  If
destruction of potential nest trees cannot be avoided, they should be
replaced with artifical nest platforms [7].  Within treeless expanses,
constructing artifical nest platforms or planting trees may also benefit
Swainson's hawk populations [6].  Additionally, establishing and
enhancing small wooded areas in the nesting habitat, and protecting
habitat used by the prey base, may benefit the Swainson's hawk [10,39].

The Swainson's hawk is more tolerant of human disturbance than other
hawks and will often nest close to occupied houses [7,14].  However,
intensive human activity in a small area near an active Swainson's hawk
nest would likely result in nest abandonment and breeding failure at
that site [7].

Swainson's hawk is sometimes eaten by people in South America.  Large
numbers are taken from communal roosting areas and killed.  This
activity could have a significant effect on populations that nest in
North America [21].  Additionally, the use of biocides in North,
Central, and South America may have an effect on Swainson's hawk
populations.  The Swainson's hawk may accumulate high pesticide levels
via food-chain concentration [31].


REFERENCES : 
NO-ENTRY

Related categories for Wildlife Species: Buteo swainsoni | Swainson's Hawk