Red-billed oxpeckers often accompany black rhinos, picking insects off the mammal's back. In Swahili, the avian companions are called Askari wa kifaru, or "the rhino's guard."

According to a new study, published Thursday in the journal Current Biology, their nickname is well-earned. Field observations suggest black rhinos with oxpeckers are better at avoiding human interactions.

Scientists surmised the avian guards act like sentinels, using calls to alert their rhino partner to the presence of potential poachers.

"Although black rhinos have large, rapier-like horns and a thick hide, they are as blind as a bat. If the conditions are right, a hunter could walk within five meters of one, as long as they are downwind," Roan Plotz, a lecturer and behavioral ecologist at Victoria University in Australia, said in a news release.

For the study, Plotz and his research partners tallied the number of oxpeckers accompanying black rhinos they encountered in South Africa's Hluh-luwe-iMfolozi Park over a period of 27 months. Because some of the rhinos were tagged, scientists knew where they were and were able to sneak up on them without being detected. Compared to the untagged rhinos scientists encountered in the wild, tagged rhinos were more likely to host an oxpecker companion.

Scientists hypothesized that untagged rhinos with red-billed oxpeckers were better able to avoid detection by the team of researchers.

"Using the differences we observed between oxpeckers on the tagged versus untagged rhinos, we estimated that between 40 percent and 50 percent of all possible black rhino encounters were thwarted by the presence of oxpeckers," said Plotz.

Even when scientists successfully detected tagged rhinos with an oxpecker companion, the bird's call alerted the rhino to the presence of the scientists. In field tests, scientists recorded the behavior of rhinos as one of their colleagues approached from a crosswind direction. Researchers noted if and when the rhino became aware of the encroaching scientist.

"Our experiment found that rhinos without oxpeckers detected a human approaching only 23 percent of the time," Plotz said. "Due to the bird's alarm call, those with oxpeckers detected the approaching human in 100 percent of our trials and at an average distance of 61 meters -- nearly four times further than when rhinos were alone. In fact, the more oxpeckers the rhino carried, the greater the distance at which a human was detected."

Scientists also observed that when rhinos heard an oxpecker call, they almost always reoriented themselves downwind, their blind spot.

The research suggests the red-billed oxpecker and their security services provide real benefits to black rhinos. Though conservation efforts have helped stabilize black rhino numbers, the mammal continues to face poaching pressure.

Some scientists estimate that the red-billed oxpecker evolved their abilities as sentinels in order to protect their source of food.

"Rhinos have been hunted by humans for tens of thousands of years, but the species was driven to the brink of extinction over the last 150 years," Plotz said. "One hypothesis is that oxpeckers have evolved this cooperative relationship with rhinos relatively recently to protect their food source from human overkill."

Unfortunately, red-billed oxpecker numbers have declined in much of southern Africa. Most black rhinos don't have avian companions, but scientists suspect efforts to reintroduce the bird to places where it's gone missing could aid the security of vulnerable rhino populations.

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