Why do elk tolerate magpies?

By Jeff Mitton

A large herd of elk had settled into Moraine Park at Rocky Mountain National Park to bask in the sun of an early-spring afternoon. They sat motionless except for their ears, which they flicked constantly.

Something was bothering their ears. Probably ticks.

A magpie glided over the herd and landed among the elk. In a determined manner, it approached a sitting elk, hopped onto its flank and continued up onto its back, where it paused to survey its surroundings. The elk did not move. The magpie climbed onto the head, and the elk remained unperturbed and motionless.

The magpie probed an ear with its bill and then stuck its head inside the ear to probe deeper. The elk did not move. Then the magpie withdrew, turned its attention to the other ear and groomed it slowly and deliberately.

The elk looked more silly than majestic with a magpie on its head, but the elk was probably more concerned with its ticks than with my opinion of its appearance.

Two more magpies arrived and began foraging on elk. A magpie started at the head, walked down the elk's neck and along the back to stand on the rump, when the elk obligingly lifted its tail. The magpie probed the edge of the tail and beneath the tail. No doubt about it, elk and magpies were cooperating to transfer ticks from the skin of elk to the bellies of magpies.

A cleaning mutualism is a mutually beneficial relationship between two species in which one removes ectoparasites, most commonly ticks, from the other. One individual is relieved of bothersome, usually blood-sucking hitchhikers and the other gains an easy meal in a safe interaction.

Several cleaning mutualisms involve birds and large mammals. The textbook example is red-billed and yellow-billed African oxpeckers cleaning rhinos, cape buffalo, zebras and giraffes. Several lesser-known mutualisms have been described in North America. Scrub jays remove ticks and insects from Columbian blacktail deer, and both scrub jays and crows groom wild boars. Magpies forage on feral horses in Nevada.

Jays, crows, ravens and magpies are all members of the family Corvidae, meaning they are closely related. Many corvids engage in cleaning mutualisms, but not all; Steller's jays and ravens are not known to groom large mammals.

Two ravens dropped into the herd and I wondered whether they would also groom elk, but they had a surprise for me and for the young elk. One of them walked up to a young elk, grabbed a bill full of fur and yanked. The elk glared at the raven, but the bold raven grabbed more fur and yanked again. The elk quickly stood up and walked away. The other raven also approached a young elk and yanked out a tuft of hair. One raven took five yanks from several elk before it could hold no more in its bill. Both ravens flew off to line their nests with elk fur.

Elk responded differently to magpies and ravens. When a magpie approached them, they sat or stood still, even when the bird probed deep into their ears. Ravens approached only young elk. Perhaps the ravens knew that the older elk had already learned to avoid ravens collecting nesting material. Each young and presumably naive elk responded to being plucked with alarm if not indignation and moved away from the aggressive raven.

Jeff Mitton (mitton@colorado.edu) is chair of the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

April 30, 2010