Posted By Bob Mason on Aug 1, 2019

July 31, 2019

By Bob Mason

Wild At Heart volunteers release rehabilitate Harris hawks back into the wild. (Courtesy Wild At Heart.)

The mental image of a pack of wolves chasing, harassing, and finally subduing their prey is a familiar one to us. Those of us who live in the Sonoran Desert have the opportunity to see this elemental demonstration of cooperative hunting. We see it in action almost every time we are observant enough to notice two or more Harris’ hawks perched on a wire or on desert vegetation.

Physical Characteristics

The Harris’ hawk is named for Edward Harris, a friend of John James Audubon, who was with Audubon when he first saw this species. The bird is properly identified using the possessive spelling and pronunciation. This hawk has many similarities to other more common raptors. It is about the size as the Swainson’s and slightly smaller than red-tailed hawks. Its wingspan, flight speed, and soaring ability compare favorably with other hawks seen here. They are primarily a bird of the dry-wooded savanna of South America but their range now extends to Arizona, New Mexico and western Texas. Unlike most hawks, it seems to thrive not only in the open desert, but also in suburban areas, where it often nests close to civilization.

When seen at a distance, the Harris’ hawk appears completely black. However, the body, breast, and head are a chocolate brown while the thighs, shoulders, and bottom of the wings are chestnut in color. The tail has a broad white band at the base and there is a narrow, white band at the tip. Like most hawks, the female is larger than the male.

These raptors may nest more than once per year. The first nesting usually begins in March or April. Two to four dull white eggs, flecked with light brown spots and about two inches long, are laid. The incubation period is 34-36 days. The chicks are fed very small pieces of meat, and when 25 days old, they are able to pull bits of meat from an animal brought to the nest by the adults.

Hunting Behavior

“Checking the Nest” by Ann Beisser, 2008 Summer Fun Contest Winner

A singular identifying behavior is their practice of hunting cooperatively. Scientists have observed that this pattern seems to be the accepted norm for this species in the Sonoran Desert. In a study reported by Natural History magazine in 1990, a group of researchers continuously monitored the hunting patterns of eight different social groups of Harris’ hawks for a period of 403 hours. During this surveillance, which included radio-telemetry, they did not observe any occasions of a “kill” by a solitary hawk.

How do these beautiful hawks arrive at such a structured system of coordinated hunting? To study this, the investigators caught, marked, and blood-tested individuals from 15 different nesting groups over a period of a year and a half. A comparison of these samples with another group of 50 hawks showed that the extra birds that hunt with each nesting pair were generally the offspring of that pair.

Another study by Professor William Mader in southern Arizona showed that all adults in the nesting group shared partners. This mating practice is called polyandry. In birds, polyandry is observed only among Harris’ hawks in Arizona and Galapagos hawks indigenous to the islands of the same name. The apparent reason for this multiple mating and their extraordinary cooperative hunting behavior is that a single Harris’ hawk, hunting alone while its mate tends the nest, cannot provide enough food to support a single pair of adults and their offspring. There is also an imbalance in the gender ratio in this species. Adult males outnumber adult females, a most unusual occurrence.

Another researcher has found that these hawks have a special fondness for jackrabbit neat. Since these hares can weigh up to three times as much as a Harris’ hawk, it is necessary for more than one hawk to cooperatively subdue this prey.

In our desert environment where the rodent and other food species are sparsely populated, it is common to find two to five Harris’ hawks hunting together. When a target animal is located and is driven by a single hawk into some kind of ground cover, a behavior signal is given to the other birds and they gather on nearby perches to circle the quarry. One bird then descends to the hiding spot to flush the prey into the open. At that point the target is surrounded and generally subdued by one or more of the group. The observers noted that the fatal strike is usually made from behind the prey while its attention is focused on another of the hunting group.

Harris Hawk

After the kill is made, one hawk usually takes up a sentinel position on a nearby high perch. The hawk making the kill is generally the first to feed, then after 15 to 30 minutes, the individuals change places.

Mammals, such as desert cottontails and pack rats, constitute about half of the Harris’ hawk diet. About 40 percent is made up of birds, such as doves, Gambel’s quail, and cactus wrens. The remainder of the diet is lizards, insects, and other invertebrates.

Falconry

In spite of their ferocious nature in the wild, Harris’ hawks are surprisingly docile where survival does not depend on killing their next meal. They are being captive-bred and are prized for their ability to be trained for the sport of falconry. These beautiful raptors and their distinctive “wolfpack” behavior can bring an added dimension to our desert landscape if we take the time to be observant.

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