The red-billed oxpecker bird perches on large mammals in southern Africa. The oxpecker perches on these mammals so that it can eat the pests that infest the other mammals. These pests often include things like ticks and fleas. It’s what biologists call a symbiotic relationship: both parties benefit from the relationship. The oxpecker gets food, and the mammal that it decides to perch on gets protection from pests and parasites.
They may also peck at wounds for fresh blood to keep the wounds open as they do prefer fresh blood - so they will also feed on ticks that have already gorged themselves on blood. This is actually where they get their name - their preference for fresh blood and their ability to peck at wounds for the blood.