In sub-Saharan Africa, red-billed oxpeckers feed on the parasites of rhinos and more than 20 other species of mammal. Now, new research suggests the birds may also serve as sentinels that help rhinos avoid humans—and potentially poachers.

Though black rhinos have a good sense of smell and good hearing, they have notoriously bad vision. If you know where one is and stay downwind of it, you can often get quite close to the animal, says Roan Plotz, now a researcher at Victoria University in Melbourne, Australia.

While Plotz was completing his doctoral thesis on black rhinos in Hluhluwe-Imfolozi Park, in eastern South Africa, he began contemplating how black rhinos might avoid humans. In recent years, poachers have decimated the ranks