It’s time for the changing of the guard. Rough-legged Hawks are on their way to the northern edge of the continent to breed and raise their young in the 24-hour summer sunshine. At the same time, Swainson’s Hawks are on their way back from Argentina in search of an endless summer.
Swainson’s Hawks were the first hawks I learned to identify as a fledgling birder. The dark-dark-light pattern of perched birds is easy to remember—dark head and chest, white belly. Plus, the russet cowl is distinctive. Smaller than other Colorado Buteos, their tiny feet allow them to perch on wires whereas other Buteos prefer trees and telephone poles. A friend described the white over their beak as their “searchlight” and the mnemonic stuck. And finally, no other American hawk has pale coverts with dark flight feathers. A soaring bird is instantly recognizable.
I get tired after a few hours in an airplane; these small hawks fly all the way here from southeastern South America. Their summer range extends from California east to the Mississippi River, and from Canada to Mexico.
While smaller than the common Red-tailed Hawk, Swainson’s Hawks have longer wings—an adaptation that helps them stay aloft for such distances. When I photographed a Swainson’s Hawk in California’s central valley last month, I became curious. Why wasn’t this bird in Argentina? It turns out that some birds only migrate as far as southern Mexico, Florida, and yes, California. (Apparently they also used to spend their winters in eastern Washington and Oregon, but not any more.)
By the time the birds return to their summer hangouts, you’d think they’d be exhausted. Maybe so, but that doesn’t stop them from immediately getting about the business of breeding. Swainson’s Hawks form permanent pair bonds, which are believed to endure as long as both birds live. Young birds and those who have lost their mates will choose a new partner as soon as they arrive on their breeding grounds. They then spend the next week constructing a nest in any tree they can find in this mostly tree-less habitat. One to four eggs are laid, and soon the parents are busy stuffing small rodents, rabbits, and snakes lizards into their hungry brood. A lot of energy goes into raising their young, and they only have one family a year.
By September the kids have fledged and formed a gang with other juveniles the same age. It’s time for everyone to head south again.
Sadly, like so many other species, Swainson’s Hawk populations are declining, and have been for the last hundred or so years. Before 1900, the species was described as abundant, but by 1910 to 1920 it had become rare over much of its range. What happened?
For one thing, farmers and ranchers routinely shot and killed Swainson’s Hawks, even though they feed on grasshoppers, locusts, and other harmful insects when not nesting. This practice finally stopped around 1930, as more enlightened views prevailed.
Additionally, the Great Plains has seen a decline in the numbers of Richardson’s Ground Squirrels, an important high protein food source for nesting birds and their young.
Insecticides are another hazard. According to Audubon’s website, “Pesticide use on alfalfa and sunflower fields in Argentina resulted in the death of some six thousand birds in 1995 and 1996. The alfalfa and sunflower fields were sprayed with organophosphate insecticides to kill grasshopper infestations. Hawks died immediately if they were sprayed while foraging in the fields or within several days after consuming the chemical-ridden grasshoppers.”
The most prevalent cause of the decline, however, is destruction of habitat. For a species easily upset by the presence of humans near its nests, urbanization eliminates large areas that once were appropriate for nesting. As one result, the population that used to live along the southern California coast has been completely wiped out.
One major limiting factor, especially on the plains, is the lack of suitable nest trees. Small-scale farmers often planted trees and large shrubs as both shelter belts and around their homes. The hawks use these plantings for and nest sites. As small farms are being incorporated into large agribusiness spreads, these trees are abandoned. Their loss is a potential threat to reproductive success.
Happily, the birds have adapted to foraging in fields planted with low-growing crops such as wheat and hay, although areas with taller crops and orchards are avoided. In fact, they’ve learned to follow tractors, pouncing on the insects and small rodents that are disturbed by tilling. Like other species, their eventual survival may depend more on their ability to accommodate to humans than the other way around.