Swainson’s Hawk

In the grand theater of North American raptors, few performers command the stage quite like the Swainson’s Hawk (Buteo swainsoni). While the Red-tailed Hawk may be the ubiquitous face of roadside fence posts, and the Ferruginous Hawk the rugged icon of the desolate badlands, the Swainson’s Hawk is the cosmopolitan traveler, the endurance athlete, and the biological bridge between the hemispheres. Known colloquially as the “Locust Hawk” for its voracious appetite for insects, this slender buteo undertakes one of the most spectacular migrations of any New World raptor, linking the open prairies of the American West with the vast pampas of Argentina.

For the serious birder and wildlife enthusiast, Buteo swainsoni represents a study in evolutionary specialization. It challenges the standard “hawk” archetype by shifting from a high-stakes hunter of vertebrates in the summer to a gregarious, insect-eating nomad in the winter. This article delves deep into the biology, ecology, and conservation of this remarkable species, moving beyond basic field marks to explore the data-driven realities of its existence.

Description

The Swainson’s Hawk is a medium-sized buteo, intermediate between the Broad-winged Hawk and the Red-tailed Hawk, but structurally distinct. It is often described as the most “falcon-like” of the buteos due to its long, tapered wings and relatively slim build. This aerodynamic profile is an adaptation for long-distance migration, reducing drag and allowing for efficient, sustained flight over thousands of miles.

Biometrics and Morphology

Unlike the bulky, broad-shouldered silhouette of a Red-tailed Hawk, the Swainson’s Hawk cuts a more elegant figure. Sexual dimorphism is present, adhering to the general raptor rule where females are larger and heavier than males, though the size disparity is less pronounced than in Accipiters.

Comparative Biometrics of Buteo Species

Plumage Polymorphism

One of the most challenging aspects of identifying Swainson’s Hawks is their extreme polymorphism. They exhibit a wide spectrum of plumage variations, typically categorized into light, intermediate, and dark morphs. Unlike Red-tailed Hawks, where polymorphism is often subspecies-related (e.g., harlani), Swainson’s morphs can occur within the same breeding population, although dark morphs are statistically more common in the western part of their range (California and Oregon) compared to the Great Plains.

Field Identification of Color Morphs

The most reliable field mark for a soaring bird, particularly the light and intermediate morphs, is the two-toned underwing. The underwing coverts (linings) are significantly lighter than the flight feathers (remiges), creating a stark contrast that is the inverse of the pattern seen in Osprey or Turkey Vultures.

Taxonomy

Scientifically known as Buteo swainsoni, this raptor was named by Charles Lucien Bonaparte in 1838 to honor the British naturalist William Swainson. Phylogenetically, it sits firmly within the Buteo genus, sharing a lineage with the Galapagos Hawk (Buteo galapagoensis). Recent molecular studies suggest a relatively recent divergence between swainsoni and galapagoensis, indicating that ancestors of the Swainson’s Hawk likely colonized the Galapagos Islands roughly 300,000 years ago—a blink of an eye in evolutionary time. This relationship highlights the dispersive capabilities inherent in the Swainson’s lineage.

Distribution, Range and Population

The Swainson’s Hawk is a bird of the open country. Its distribution is bifurcated into two distinct, seasonally occupied hemispheres.

Breeding Range: The species breeds across the western half of North America. Its stronghold is the Great Plains, extending from the Canadian prairies (Alberta, Saskatchewan, Manitoba) south through the Dakotas, Nebraska, Kansas, and into Texas. A separate, somewhat disjunct population exists west of the Continental Divide in the Great Basin, the Central Valley of California, and parts of Oregon and Washington.

Wintering Range: The “Pampas” region of Argentina is the primary wintering ground for nearly the entire global population. Smaller numbers winter in Uruguay and southern Brazil. A fascinating, small localized wintering population has recently established itself in the Sacramento-San Joaquin River Delta of California and Southern Florida, potentially responding to warming climates and agricultural changes.

Global Population Estimates and Trends

Data synthesized from Partners in Flight and USGS Breeding Bird Survey.

While the global population appears robust, local extirpations have occurred. The species has largely vanished from its historic breeding range in southern California and parts of the Pacific Northwest, primarily due to the conversion of native grassland and alfalfa fields to orchards and vineyards, which do not support the hawk’s hunting style.

Habitat

The habitat requirements of the Swainson’s Hawk are characterized by a “landscape of contrast.” They require wide-open foraging areas for hunting but rely on vertical structures for nesting. This duality makes them highly adaptable to certain agricultural landscapes but vulnerable to others.

• Breeding Habitat: Originally, the shortgrass and mixed-grass prairies were their domain. Today, they have adapted to agricultural mosaics. They thrive in areas with row crops (alfalfa, hay, wheat) interspersed with shelterbelts, riparian corridors, or isolated trees. They avoid dense forests and completely urbanized areas.

• Foraging Requirements: Vegetation height is the critical variable. Studies show a strong preference for foraging in hay and alfalfa fields during harvest (mowing), which exposes prey. They avoid tall, dense crops like mature corn or sunflowers where prey visibility is low.

Nest Site Selection Preferences

Behavior

The Swainson’s Hawk is a study in social contradiction. During the breeding season, pairs are territorial, defending a radius around their nest from conspecifics and other raptors (especially Red-tailed Hawks). However, once the breeding switch is flipped off, they become one of the most gregarious raptors on earth.

The “Kettle” Phenomenon

During migration, tens of thousands of Swainson’s Hawks aggregate into massive flocks known as “kettles.” Utilizing thermal updrafts to gain altitude, they swirl in a chaotic yet synchronized vortex. This sociality extends to their wintering grounds in Argentina, where roosts can contain thousands of birds. This behavior is likely an adaptation to maximize information transfer regarding ephemeral food sources (locust swarms) and to utilize limited roosting sites in the open pampas.

Flight Style

In flight, the Swainson’s Hawk holds its wings in a distinct dihedral (V-shape), similar to a Turkey Vulture or Northern Harrier, though less pronounced. This creates stability during low-speed soaring and coursing. Their flight is buoyant, often described as “teetering” in the wind, a trait that helps distinguish them from the steadier, flatter soaring profile of the Red-tailed Hawk.

Flight Silhouette Comparison

Feeding

The dietary flexibility of the Swainson’s Hawk is its superpower. It exhibits a complete trophic shift between seasons, essentially acting as a raptor in the summer and a high-volume insectivore in the winter.

Breeding Season: The Vertebrate Hunter

While raising young, energy demands are high. The hawks focus on protein-rich vertebrates. In the Great Plains and Great Basin, their diet is synchronized with the reproductive cycles of small mammals.

• Key Prey: Richardson’s Ground Squirrel, Thirteen-lined Ground Squirrel, Pocket Gophers, Voles, and Deer Mice.

• Secondary Prey: Reptiles (snakes, lizards) and ground-dwelling birds (meadowlarks, pheasant chicks).

Non-Breeding Season: The “Locust Hawk”

During migration and wintering, the diet shifts almost 100% to invertebrates. They follow farm machinery or insect swarms, often running awkwardly on the ground to catch prey.

• Key Prey: Grasshoppers, crickets, locusts, dragonflies, and beetles.

Dietary Shift Breakdown (Biomass estimates)

Note: While insects make up a small percentage of biomass during breeding, they may make up a high percentage of individual items captured.

Breeding

Swainson’s Hawks are monogamous with high mate fidelity, often returning to the same nesting territories year after year.

• Arrival: Males typically arrive first in late March to mid-April to stake claims.

• Nest Construction: The nest is a flimsy platform of sticks, often lined with fresh green leaves or grass. This “greenery” behavior is thought to signal territory occupancy.

• The Cycle:

  • Clutch: 2-3 eggs (rarely 4). Eggs are pale bluish-white with brown spotting.

  • Incubation: 34-35 days, primarily by the female. The male provides food.

  • Fledging: Young leave the nest at 42-44 days but depend on parents for another 3-4 weeks.

A critical data point in their breeding biology is brood reduction. In years of low prey density (e.g., ground squirrel crashes), the youngest chick often starves or is killed by siblings (cainism), a brutal but effective adaptation to ensure that at least the strongest offspring survive.

Reproductive Success Rates by Habitat

Threats

Despite being categorized as “Least Concern” globally, the Swainson’s Hawk faces distinct, high-impact threats.

The Argentina Poisoning Crisis (Monocrotophos)

In the mid-1990s, researchers noticed a catastrophic decline in returning hawks. Tracking revealed a horror story on the wintering grounds. Farmers in Argentina were using monocrotophos, an organophosphate pesticide, to control grasshoppers in sunflower and alfalfa fields. Because Swainson’s Hawks gorge on these insects, they were dying en masse.

• Data Impact: In the winter of 1995-96 alone, an estimated 20,000 to 30,000 hawks died (roughly 5% of the world population).

• Resolution: A massive international campaign led to the banning of monocrotophos in Argentina for use on crops frequented by the hawks. Populations have since stabilized, but the event underscores the vulnerability of migratory species.

Habitat Conversion

In the breeding range, the loss of nesting trees and the conversion of alfalfa/hay fields (high quality) to vineyards, orchards, or urbanization (low quality) is a primary driver of local declines, particularly in California.

Migration

The migration of the Swainson’s Hawk is the stuff of legend. It is a hemispheric traveler, clocking round-trips of 12,000 to 14,000 miles.

• The Route: Birds from across North America funnel through Mexico and Central America. Unlike broad-front migrants, they concentrate into a narrow “River of Raptors” through the Isthmus of Tehuantepec in Mexico and Panama.

• Veracruz Count: At the Veracruz River of Raptors monitoring site in Mexico, observers regularly count over 1 million Swainson’s Hawks in a single season, often with single-day counts exceeding 100,000 birds.

• Fasting: Evidence suggests that during the sprint through Central America, the hawks may not feed for days, relying on fat stores built up in the Great Plains.

Migration Statistics

Conclusion

The Swainson’s Hawk is more than just a bird; it is a thread that stitches together the ecosystems of two continents. From the shelterbelts of North Dakota to the alfalfa fields of La Pampa, Argentina, its presence is a barometer for the health of our grasslands and agricultural systems. While they have recovered from the acute threats of the 1990s, they remain reliant on a delicate balance of land use and international cooperation. For the birder standing on a dusty plain, watching a kettle of these birds spiral into the ether, the Swainson’s Hawk offers a profound connection to the global scale of nature.

Additional Notes: Adaptations and Culture

• Physiological Adaptation: Swainson’s Hawks have evolved to efficiently digest a diet of pure chitin (insect exoskeletons) during winter, a metabolic flexibility rare in large raptors.

• Cultural Significance: Farmers in the early 20th century often protected these birds, recognizing their value in pest control before chemical pesticides became widespread. A family of Swainson’s Hawks can consume thousands of rodents and hundreds of thousands of insects in a single season, providing a free ecosystem service worth thousands of dollars.