Hawks stoop with a group to increase hunting success.
Tag team competition extends beyond the professional wrestling arena. Groups of Harris' hawks in New Mexico use a variety of cooperative hunting techniques, including a "relay attack," to capture rabbits and hares.
Cooperative hunting, in which many animals team up to capture and share one large prey, has long been observed in a small collection of mammalian species including lions, hyenas and wolves. But this is the first documented observation of such coordinated hunting behavior in birds, reports James Bednarz in the March 25 issue of SCIENCE. "There have been reports of pairs of falcons hunting together during mating season, but the pairs weren't very cooperative or successful," says Bednarz, who did the work while at the University of New Mexico in Albuquerque and is now a staff member of the Hawk Mountain Sanctuary Association in Kempton, Pa.
Harris' hawks are large hawks that normally survive by eating quail and other small birds. The hawks eat much better if they can capture rabbits or hares, but it is very difficult for a single hawk to do so because the speedy rabbits outweigh the hawks by as much as 3 to 1. Bednarz found that groups of four to six hawks increased their chances of capturing rabbits by employing group hunting techniques to seek out, then tire and confuse their prey.
The most common tactic was the "surprise pounce," in which many different hawks would dive at the rabbit from different directions, confusing it until one hawk could get a clean shot at the animal. In the "relay attack," the hawks chased the rabbit for several minutes; when the leader dived at the prey and missed, another assumed the lead position.
The "flush and ambush" strategy often was invoked if the rabbit found refuge in a bush or copse of trees. One hawk would land and walk into the cover, flushing out the prey and exposing it to the waiting hawks outside. The hawk on the ground has almost no chance of capturing the rabbit on its own, Bednarz explains, so it is acting to increase the group's chance of success rather than just its own.
The "flush and ambush" tactic is a strong example of how cooperative hunting differs from social hunting, Bednarz says. In social hunting, animals hunt in the same place because the presence of the others increases the individual's chance of success, he explains. Pelicans are hunting socially when they surround a school of fish to increase the individual's chance of catching one, Bednarz says.
The New Mexico Harris' hawks, on the other hand, work together to increase the group's success rather than the individual's. In most cases, Bednarz found, a hawk will dive at a rabbit even when there is no chance of catching the animal, just to cut off its escape. "For instance, a hawk will never catch a rabbit by coming at it head-on, but the hawk will come at the rabbit that way to turn it around and keep it in the field of play," Bednarz says.
Bednarz observed the hawks by using radio transmitters attached to the birds to follow them. Every 30 seconds he and his assistants marked the position and activity of the tagged bird and made notes about the activity of the group the bird was with. "I think we were able to observe this sort of behavior because we were maintaining constant surveillance of individual birds, which has not been done too often," Bednarz says.
The ability to hunt cooperatively is probably able to develop because the New Mexico population doesn't migrate and has a stable social structure, and because the rabbits can offer an important food source, in this case accounting for almost 90 percent of the hawks' energy needs. Individual hawks have almost no chance of getting those needed calories, because the adult rabbits' speed, size and powerful hind legs don't leave the furry creatures defenseless. "I have no doubt that a rabbit's kick could break a hawk's bones," Bednarz says, "and the hawks are very cautious when attacking."
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|Title Annotation:||cooperative hunting techniques|
|Date:||Apr 2, 1988|
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