How does the backward-facing hallux help passeriforms grip a perch?

The hallux, or hind toe, is essential to the passeriform foot because it is positioned on the same level as the three front toes and acts in opposition to them. This arrangement allows the bird to effectively grasp and secure itself onto a perch.1

To facilitate this grip, the bird utilizes deep flexor muscles located in the leg. These muscles are connected to the toes by deep plantar tendons that extend down the back of the tarsometatarsus. When these muscles contract, the tendons pull the toes closed, creating a firm hold on a branch or other surface. While most passerines possess this specialized perching foot, some species that have adapted to terrestrial life exhibit modifications, such as longer, straighter claws on the hallux to assist with balance while walking or running.21