Swainson’s Hawk
By: Colleen Del Vecchio, Robert Shields, and Geoff Cline
Do you ever dream of an endless summer? One bird, the Swainson’s hawk (Buteo swainsonii), gets to live that dream by migrating between the northern and southern hemispheres and chasing the warm, sunny weather.
Swainson’s Hawk on a fence post. Photo by: Robert Shields
What is a Swainson’s Hawk?
Swainson’s hawks are members of the Accipiter family, which includes hawks, eagles, vultures, and kites. They have a tall, slim body with broad wings, which provides them with the aerodynamic shape needed to migrate long distances while retaining the ability to hunt darting prey. They may have some of the most variable feather colors of all hawks. Variations found in this species can range from light to dark feather patterns, or “morphs (Wheeler 2018).” Generally, they have light-colored bellies, a dark or reddish-brown chest, white wing linings with dark flight feathers, and a white patch around their beak (Bechard 2020).
Are you in the vicinity of Sacramento or Fresno and want to see one? If your answer is yes, they can be found in the spring and summer months foraging in alfalfa fields, other agricultural fields, and grasslands within the Central Valley of California, and other parts of North America. Their diet during the breeding season (generally March through July) primarily consists of voles, mice, ground squirrels, and rabbits, but they are also known to prey on insects, lizards, snakes, birds, and bats (Bechard 2020). During the non-breeding season (generally August through February) their diet is mostly limited to insects, such as crickets, grasshoppers, dragonflies, butterflies, and other flying insects (Bechard 2020). They typically hunt from a perch by waiting patiently for the right moment to seize their prey or sometimes they run along the ground and pounce when the moment is right (Bechard 2020). This is always a comical scene, watching a large bird of prey hopping along the ground and jumping on insects. They can also be seen catching insects in the air while soaring, also known as “hawking.” Swainson’s hawks have relatively small talons compared to other hawks, which scientists believe assist them with catching insects.
A young Swainson’s hawk sitting in a nest. Photo by: Nathan Hale
The bountiful lands in North America provide enough food for Swainson’s hawks to raise their young. They prefer to nest in trees along rivers and creeks but are also known to nest in trees in highway medians. They have strong nest fidelity and will return to the same nest to breed each spring, as long as another raptor is not using it. Once the chicks hatch, fledge the nest, and begin to forage on their own, the weather begins to cool, and most individuals get the itch to migrate to the warmer weather in South America.
Each year, most Swainson’s hawks migrate between North and South America, with some Swainson’s hawks migrating distances equal to nearly half the circumference of the earth. At the farthest points of their migration routes, they travel from breeding grounds in Alaska and northern Canada to their non-breeding grounds in Argentina, a distance of up to 6,000 miles (Bechard 2020). They are known to travel in large flocks or “kettles” during the day and use thermals or rising hot air to soar using minimal energy. On their migration path through Central America, they fly through much narrower lands. These bottlenecks result in large, dense flocks. In Veracruz, Mexico, the Veracruz River of Raptors Project found that over 100,000 Swainson’s hawks migrate in the spring and over 800,000 migrate in the fall (Inzunza 1991). Remarkably, not all Swainson’s hawks migrate out of the United States. There are some small non-migratory populations that remain in the delta region of California and the southern coast of Florida.
Swainson’s hawk in an open field. Photo by: Robert Shields
What are their threats?
In California, the Swainson’s hawk is listed as a threatened species by the California Department of Fish and Wildlife due to their relatively low numbers. They are protected by the federal Migratory Bird Treaty Act, California Endangered Species Act, and California Fish and Game Code. In California and other parts of North America, their biggest threat is development, which causes loss of foraging habitat and nest trees. They require approximately 6,000 to over 9,000 acres of foraging habitat in order to find enough food to raise their young (Estep 1989 and Babcock 1995). When this habitat is lost due to development or conversion of these lands to other uses, it chips away at their resources, which may result in an area that no longer has enough foraging habitat within the vicinity of their nest tree. In addition, when their nest trees are cut down and they lose their nest, they need to find a new tree and build a nest. Or, if their nest trees are cut down during the breeding season, their offspring will likely die.
Since Swainson’s hawks are such long-distance migrants, another large threat to them is the lack of law enforcement and the allowance of unsafe insecticides used in agriculture operations in Central and South American countries. In the mid-1990s, Argentina had a grasshopper plague that was controlled using an insecticide, which resulted in a mass mortality of Swainson’s hawks. Scientists estimated 20,000 individuals were killed by direct contact of being sprayed by this insecticide or indirectly by consuming poisoned grasshoppers (Bechard 2020). This chemical (monocrotophos) was banned in Argentina in 1999, but other highly toxic and unregulated insecticides are still being used today in Central and South America (Bechard 2020).
What kind of conservation efforts are being completed?
You may ask yourself, what do Swainson’s hawks do for humans? Swainson’s hawks are an important species in the food chain that regulate pest species occurring in our agricultural lands. Without them, there would be larger losses of crops and more pests. Conserving them is important for the ecosystem and us.
Because Swainson’s hawks are a threatened species in California, a majority of the conservation work occurs here. When new developments are proposed and must go through the environmental review process, impacts to their foraging habitat and nest trees are analyzed. Mitigation measures would be proposed if the project may impact a nest. Some mitigation measures may include establishing a no-disturbance buffer around an active nest and removing the nest tree outside of the nesting season and replacing the tree with other potential nest trees elsewhere. Larger projects analyze cumulative effects of their project and other projects in the vicinity to determine if there would be any significant impacts to Swainson’s hawk foraging habitat. In some cases, the California Department of Fish and Wildlife may require a project owner to purchase and fund the maintenance of conservation lands with suitable foraging habitat for Swainson’s hawk to offset the impact of the project.
Swainson’s hawk eating a meal. Photo by: Robert Shields
Other conservation efforts include delivering education programs in Argentina to reduce the use of the insecticides that are known to kill Swainson’s hawks, avoiding impacts by completion of protocol-level surveys to determine their presence or absence, and growing certain crops to support foraging Swainson’s hawks during their breeding season.
Fun Facts
eBird has collected all of the Swainson’s hawk locations from its users from 2006 to 2020 to create an abundance animation map that shows you their movement month to month as they migrate through the Americas.
Fledgling (young that recently left the nest) Swainson’s hawks feed almost exclusively on insects for the first few months.
Dark morph Swainson’s hawks only breed in Western North America (Wheeler 2018).
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