Question

why some bird sits on the back of a bigger animal​

Answer 1

Explanation:

To take rest and get some warmth from the bigger animal's body.

Answer 2

Big animals tend to attract flies, fleas, and ticks, so for an insect-eating bird, they're like a buffet. They're also a safe place to perch to avoid small predators like foxes and cats, who generally won't mess with large, hoofed creatures that could wreck them with one kick.