Birds ride on rhinos and buffalo all day, picking parasites off like a grooming service

Oxpeckers are African birds that spend much of their lives perched on large mammals like rhinos, giraffes and buffalo, eating ticks and parasites straight off their hosts' skin. Long held up as a textbook example of mutualism, the relationship turned out to be more complicated: field research found oxpeckers also peck at open wounds and drink blood. Hosts still tend to tolerate them for the pest control and the alarm calls they give when predators approach.

— Paul Weeks, Red-billed Oxpeckers: Vampires or Tickbirds? — Behavioral Ecology, 2000