Which bird can climb down tree trunks headfirst?

A nuthatch can climb down tree trunks headfirst.

Nuthatches are compact woodland birds in the family Sittidae, recognized for moving in quick hops over bark. Their strong legs, sharply curved claws and flexible ankle joints let them descend vertically with the head pointing toward the ground. They can also hang upside-down beneath branches while searching for prey.

This behavior helps nuthatches inspect tree bark from an angle many other birds cannot manage. They pick out insects and other invertebrates hidden in crevices, then use their sturdy bills to extract or hammer apart food. Many species also wedge seeds, acorns or nuts into cracks and return to them later.

Treecreepers and woodpeckers are common alternatives in this question, but their climbing styles differ. Treecreepers generally move upward and brace themselves with their tails, while woodpeckers commonly use their stiff tail feathers as a prop. Nuthatches rely mainly on their feet and claws instead.