The Swainson’s hawk is a medium-sized raptor that breeds in western North America and migrates to southern South America for the winter. Its common name commemorates the English naturalist William Swainson. Measuring 46–58 cm in length with a wingspan of 120–150 cm, the Swainson’s hawk is a slim, long-winged buteo. Their breeding habitat is prairie and desert grasslands in western North America. They nest in isolated trees or bush, usually where there are abundant rodent populations to feed the young.
These hawks migrate over 11,000 km each way to wintering grounds in southern South America. The Swainson’s hawk forms social groups during migration for food and protection.
Nesting Range and Habitat
The Swainson’s hawk breeds from Alaska and northern Canada south to northern Mexico, and migrates to southern South America for the winter. Its breeding habitat is open prairie and desert grasslands with scattered trees or shrubs.
They build nests in isolated trees, bushes, utility poles, or other elevated structures. Nest sites are often located near rodent populations which provide an abundant food source. Nests are generally constructed at a height of 3-15 m. The same nest site may be reused in subsequent years.
Nest Construction
Swainson’s hawks begin building nests in late March or April at the start of the breeding season. The nests are built primarily by the female with some assistance from the male.
Nests are constructed of sticks, branches, and lined with grass, bark strips, feathers, and other soft materials. Swainson’s hawk nests are generally 60-90 cm wide and range from 15-60 cm deep. The nests are sturdy enough to support incubating adults and withstand harsh weather.
Egg Laying and Incubation
The female Swainson’s hawk typically lays 1-4 eggs, with 2-3 being most common. The eggs are whitish in color with brown spotting and measure about 60 x 46 mm. The eggs are laid over a span of several days in late April or May.
Incubation begins when the first egg is laid and lasts about 35 days. The female does the majority of incubation while the male provides food. During incubation, the female will leave the nest for short periods to feed, preen, or defecate. The male closely guards the nest during the female’s absence.
Hatching and Nestlings
The eggs hatch asynchronously over 2-4 days in late May or early June. Upon hatching, the nestlings are semi-altricial, covered in white down. Their eyes are open at hatching.
The female broods and feeds the young while the male hunts. Nestlings are fed chopped up rodents, birds, reptiles, and insects. As they grow, the nestlings develop dark feathering within 1-2 weeks.
By about 35 days old, the young are capable of making short flights near the nest. They fledge and leave the nest for good at 6-7 weeks old in early to mid July. However, the fledglings continue to be dependent on their parents for food for another 4-6 weeks.
Parental Care
Both parents share responsibility for feeding the young, though the male does the majority of the hunting. The female tears up prey items into small pieces suitable for the nestlings. As they get older, the nestlings are fed larger chunks of prey.
The parents are vigilant in defending the nest against potential predators. They may harass intruders by diving at them or even striking with their talons. Parental care continues for about a month after fledging until the young hawks can hunt proficiently on their own.
Migration
In August and September, Swainson’s hawk families leave the breeding grounds and migrate south. Their wintering range consists of open grasslands and agricultural areas from southern South America to central Argentina.
Swainson’s hawks migrate during the day in flocks of 10 to over 1,000 individuals. These flocks provide protection from predators during migration. Swainson’s hawks utilize thermals and updrafts to gain altitude and glide over long distances.
The northward spring migration occurs over a broader time period from February to May. The hawks follow the same migratory routes and use the same stopover sites each year. Successful adults exhibit high nest site fidelity, often returning to breed in the same area year after year.
Diet
Swainson’s hawks are opportunistic foragers and hunt by searching for prey from a perch or soaring flight. Their diet is diverse, consisting mainly of small mammals, birds, reptiles, amphibians, and large insects.
Mammals make up a large portion of their diet during the breeding season when rodent populations are high. Voles, ground squirrels, mice, rats, rabbits, shrews, and gophers are commonly taken. Birds such as starlings, blackbirds, doves, and sparrows are also preyed on frequently.
Insects including grasshoppers, crickets, cicadas and dragonflies become important food sources while on their South American wintering grounds. Prey is generally caught on the ground or in open aerial pursuits. The hawks use their talons to dispatch and carry prey to a perch for eating.
Threats and Conservation
The global population of Swainson’s hawks is estimated at around 775,000 individuals. Their numbers declined during the 20th century, though the population has been increasing in recent decades.
Major threats include habitat loss on breeding and wintering grounds, as well as pesticide use impacting prey populations. Collisions with vehicles and powerlines also result in many hawk deaths annually.
Conservation efforts for Swainson’s hawks have included protecting grassland habitats, setting restrictions on pesticide use, and modifying utility structures to prevent electrocution. With continued habitat protection and reduced human impacts, this widespread raptor remains common over much of its range.
Unique Aspects of Swainson’s Hawk Nesting Behavior
Some unique aspects of Swainson’s hawk nesting behavior include:
– Extremely long distance migrants – they travel up to 18,000 km round trip each year between breeding and wintering grounds.
– Form large flocks during migration – groups of over 1,000 individuals have been observed migrating together.
– Hunt in a “follower” strategy – multiple hawks will follow grass fires or farm machinery to catch prey flushed by the disturbance.
– Opportunistic, generalist diet – they prey on whichever small vertebrates are most abundant, switching from mainly insects in winter to small mammals in summer.
– Show high nest fidelity – pairs often return to the exact same nest site for multiple years in a row.
– Late summer/fall migration – most raptors migrate earlier, but Swainson’s hawks migrate south in late summer/early fall.
– Gregarious at nest sites – nests are often semi-colonial with multiple active nests in the same grove of trees.
– Predominantly grassland nesters – most hawks are forest nesters, but Swainson’s favor open grasslands for nesting habitat.
Conclusion
In summary, the Swainson’s hawk is a long-distance migratory raptor adapted to breeding in open grasslands and agricultural regions of western North America. They build sturdy stick nests at heights of 3-15 m in lone trees or shrubs. Females lay 2-4 eggs in April-May, incubate them for 35 days, and brood newly hatched young.
The semi-altricial nestlings are fed by both parents and fledge in 6-7 weeks in early to mid summer. Swainson’s hawks form social flocks during migration and exhibit high fidelity to breeding and wintering sites between years. They are opportunistic foragers that prey mainly on small mammals, birds, reptiles, and insects. Continued habitat protection and management will be important for conserving populations of this widespread raptor into the future.