Do oxpeckers help or mostly just freeload?

Birds that ride around on large African mammals picking off ticks provide a common textbook example of mutualism, but the animals’ interactions may not exemplify such a happy partnership after all.

Keeping the red-billed oxpeckers away from oxen didn’t typically increase tick infestation, Paul Weeks found in graduate work for the University of Cambridge in England. Weeks also observed that excluding the oxpeckers speeded healing of skin wounds of various causes, he reports in the March-April issue of Behavioral Ecology.

“When people see animals together, they like to think nice things about them,” he says. Yet evidence that the birds provide genuine pest control has largely been indirect, Weeks notes.

The birds provide a dry-land counterpart to the other much-discussed vertebrate mutualism, small fish nibbling parasites off bigger species at cleaning stations on reefs. Last year, a researcher in Australia finally demonstrated that the cleaning reduces parasite numbers.

Weeks focused on oxpeckers riding a herd of 22 Bonsmara oxen in Zimbabwe. He says the birds spent less than 15 percent of foraging time eating adult ticks. Otherwise, they fed on skin wounds and ear wax or scissored their bills through the hair, perhaps to find small tidbits.

He divided the herd and assigned an assistant to chase away oxpeckers from one group for a month. In three trials, he found no consistent increase in the pests among the birdless oxen.

In unpublished work, he also tested the idea that oxen benefit from bird alarms. Oxpeckers called at hawks, people, or other threats to birds, Weeks says. Yet when a helper in a lion skin crept forward, with a baby buggy to support the head, the cattle spooked but the birds kept eating.

Weeks worked with ranch oxen but says he suspects that the birds aren’t much help to the 25 wild animal species with which they associate.

Walter D. Koenig of the University of California, Berkeley says that for wild animals, “my guess is that oxpeckers may indeed forage on ear wax, blood, and the like a lot more than previously thought, but still a lot less than they do on parasites.” Nevertheless, he applauds Weeks’ work as the first experimental study of oxpeckers.

Mutualism specialist Judith Bronstein of the University of Arizona in Tucson has long been skeptical of vertebrate partnerships involving a cleaner. She says, “I can easily believe that oxpeckers are not beneficial to oxen.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.