The birds on their backs are a distant early warning system for endangered rhinos

In the Swahili language, the red-billed oxpecker bird is called Askari wa kifaru, which translates to "the rhino's guard." Scientists have now figured out exactly how the bird stands guard — scientists have observed the birds warning black rhinoceroses of approaching threats.

Rhinos need all the help they can get according to Roan Plotz, a lecturer and behavioural ecologist at Victoria University in Melbourne, Australia. 

Black rhinos are an endangered species, poached by humans for their horns. While their numbers have increased slightly due to conservation efforts in recent years, there are just over 5,000 remaining in the wild. 

Rhinos have extremely poor vision. Poachers are able to approach within about five metres of a rhino as long as they're downwind, according to Plotz.

The rhino makes up for its poor eyesight by having an ally in the red-billed oxpecker.

The bird rides on the rhino's back, feeding on ticks and parasitic worms embedded in lesions on the giant herbivore's body. 

The bird also does this with other large African mammals, but there is a big difference when it comes to the rhino. Other mammals try to get rid of the bird when it picks at lesions — not so with the rhino. 

Studying tagged black rhinos in South Africa's Hluhluwe Imfolozi Park, Plotz and his colleagues observed that they carried one or more oxpeckers most of the time.

Their research revealed that the alarm call of the oxpecker was a valuable safety cue for the black rhino.

The researchers designed an experiment in which one member of the team walked toward a rhino in a crosswind, while another researcher recorded the resulting behaviour.

After a total of 86 approaches, Plotz and his colleagues found that rhinos without oxpeckers on their backs detected the approaching human 23 per cent of the time.

However, rhinos with oxpeckers onboard detected the approaching human 100 per cent of the time, and from an astounding distance of 61 metres on average. Human approach without oxpeckers averaged 27 metres. 

As soon as the oxpeckers sound the alarm, the rhino immediately turns to orient itself downwind. This is because rhinos smell better than they see, and poachers approach the animals from downwind to avoid detection by scent. 

Plotz thinks this behaviour may be relatively new. 

Rhinos have been hunted for tens of thousands of years but were driven to the brink of extinction with increased hunting pressure that began in the 1850s. This could be a response to that increased pressure, according to Plotz.  

As for the oxpecker, it is simply protecting its source of food.