For the Birds Radio Program: Commensalism
Reworked from 2-1-88. (3:52)
(Recording of a Canada Jay)
One of the big advantages of being a birdwatcher is that you understand references to birds in literature that go over regular people’s heads. Long ago, when I read Jack London’s classic White Fang to my little boy Joey, I was perfectly qualified to explain what a ptarmigan is when White Fang met up with some ptarmigan chicks and their mother. Joey had had enough personal experience with hawks at Hawk Ridge that he didn’t need any explanation when a large hawk buzzed White Fang his first day out of the lair. But when I came to references about a moosebird, I was stumped–for once I had to look it up. It turns out a moosebird is a Gray Jay–also known as a Canada Jay or a Whiskey Jack. It got the nickname Moosebird because it often sits on the backs or antlers of moose, picking off ticks and deerflies or just hanging on for the ride.
This kind of unwritten agreement that benefits two different species is called commensalism. There are many examples of commensalism in the bird world. Perhaps the most well known is the Cattle Egret. The Cattle Egret is a native of Africa, which found its way to Central or South America in the late 19th century. The birds slowly expanded northward– the first breeding record in the U.S. was by seven pairs in Florida in 1952. Ten years later this little group became a colony of about 4,000, and within the next ten years this species became the most abundant egret in North America. There have been breeding records of Cattle Egrets in Minnesota and Wisconsin, and they’ve been sighted as far north as Grand Marais.
Many African legends and folktales explain why these lovely white birds sit on the backs of water buffalo, cattle, and other large mammals. Scientists prefer the prosaic explanation that they do it for food–Cattle Egrets pick at insect parasites on their hosts, and also eat earthworms and other soil invertebrates on the ground which are exposed when large hoofed mammals walk along. Some gulls do the same thing in the Great Plains. Not only do gulls follow buffalo and cattle–they’ve also learned to follow tractors for the earthworms kicked up. And the cowbird received its name from hanging around cows. Before European man completely altered the American environment, cowbirds were restricted to the Great Plains, where they lived in close association with American bison. But at the same time the bison were extirpated, eastern forests were being chopped down and cattle brought in, and now cowbirds can be found just about anywhere in the U.S.
Tickbirds in Africa groom buffalo, rhinoceros, giraffes, and other mammals. And the Ground Finch of the Galapagos Islands pulls ticks off marine iguanas and giant tortoises. The tortoises crane their necks and stretch high on their legs, remaining motionless as long as five minutes while the finches pull out ticks from the crevices in their skin. That way both species come out ahead.
Birds also have commensal relationships with other birds. Many songbirds, especially grackles, nest in the bottom sticks of eagle and osprey nests. The grackles eat dropped food particles from the raptors, which not only benefits the grackles, it also may keep the nest’s bacteria levels down for the eagles and ospreys.
Probably the most satisfying commensal relationship is between birds and humans. When birds come to feeders, their hunger is satisfied at the same time that the human’s drives to nurture and to find aesthetic amusements are satisfied.
(Recording of a Black-capped Chickadee)
This is Laura Erickson, and this program has been “For the Birds.”