麻豆传媒

(Image: Charles W. Juels. Courtesy of Anacortes Telescopes, www.buytelescopes.com

Zoologger is our weekly column highlighting extraordinary animals 鈥� and occasionally other organisms 鈥� from around the world

Species: Harris鈥檚 or Harris hawk (Parabuteo unicinctus)
Habitat: Arid scrublands from the south-western US down to Chile and Argentina

Meet the only raptors known to hunt in packs. Some Harris鈥檚 hawks work in teams with a whole playbook of tricks 鈥� and if you鈥檙e a rabbit or a lizard, they鈥檙e the last things you want on your tail.

鈥淚t鈥檚 amazing to watch,鈥� says ecologist of the Orleans Audubon Society in Pearl River, Louisiana. Coulson is hoping to understand how cooperative hunting works in the wild, where it鈥檚 been observed in groups of two to nine wild Harris鈥檚 hawks. But she and her husband, both avid falconers, also have their own trained hunting team, which allows them to see such behaviour up close.

First there鈥檚 鈥渂ackstanding鈥�, which looks how it sounds. 鈥淭wo or three hawks will stand on each other, so they look like a hawk totem pole,鈥� Coulson says. The hawk on top, which is careful to not hurt its living perch by digging in its talons, gets a better vantage point when scanning for prey.

Share and conquer

As they hunt, individuals take on roles that are suited to their strengths or their status in a dominance hierarchy 鈥� as is seen in wolves. Some hawks, blessed with better vision, become the group鈥檚 prey spotters, Coulson says. If they take wing to chase a rabbit, the others follow.

Once airborne, a single hawk will lead the pursuit, with wingmen on either side that can visually pick up the prey if it turns sharply. If the first hawk dives and misses, another will slide into place on high, ready to take another shot while the first hawk gets back into line. It鈥檚 like a whole quidditch team bent on making a kill.

The teamwork doesn鈥檛 end there. If prey goes to hide under brush, certain hawks act as 鈥渇lushers鈥�, who will sacrifice their own chance at a kill. Instead, they fly down noisily to wade through the brush, scaring the animal out back into the open. In other situations, such as when a pack rat seeks safety in its nest, 鈥渂locker鈥� hawks will stick their feet down the hole to plug or guard that exit.

What makes this collective approach work is that these uniquely social hawks share their kills afterward. 鈥淐ooperation in predators is always fascinating to me because it鈥檚 a dog eat dog 鈥� or a hawk eat hawk 鈥� world out there,鈥� Coulson says. Any member of the team could sneak up on and kill another member, or at least steal more than their fair share.

Whether in hawks, chimps, lions, sharks, groupers or wolves, cooperative hunting should only be able to evolve when the benefits outweigh the risks and the cost of going it alone. But we still don鈥檛 know why Harris鈥檚 hawks in the southern US do it 鈥� or even whether the South American subspecies, which is rarely seen in groups, does it at all.

Flat-sharing birds

Theories for why team hunting works for some Harris鈥檚 hawks are legion. It could be that it allows them to take down bigger prey because it gives the group a better chance of snagging at least one large animal in the short windows of good hunting at dusk and dawn. Or it could be because they can better protect their kills from scavengers. Perhaps the young that stick around and pitch in on hunts are more likely to survive.

Coulson鈥檚 own on the subject argues that group hunting becomes necessary because of how tough some Harris鈥檚 hawk habitats are. Although any of the above may also contribute, she thinks that in landscapes with few prey animals and lots of cover to hide under, groups might be better off than lone hawks or even pairs.

The paper has been slow to kickstart the discussion, but Coulson is hopeful for an eventual answer. Ongoing surveys of South American raptors may extend our knowledge of the environments in which Harris鈥檚 hawks cooperate and allow for comparison between the different theories.

More observations of trios of adult Harris鈥檚 hawks foraging together in areas not generally known for group hunting were reported in August, at the joint conference of the American Ornithologists鈥� Union and the Cooper Ornithological Society at the University of Oklahoma. That鈥檚 important because the hawks don鈥檛 just hunt together 鈥� they also live and breed in shared homes. It isn鈥檛 yet clear whether cooperative breeding leads to cooperative hunting, or vice versa, but understanding both in context will be key, says Coulson.

鈥淲hen I lecture about cooperation in Harris鈥檚 hawks, my presentation title is 鈥楾he family that preys together, stays together鈥�,鈥� she says.

Journal reference: Auk, DOI: