Which toe arrangement helps passeriform birds perch on branches?

Passeriform birds possess a specialized perching foot characterized by an arrangement where all four toes are well-developed and free from one another. The hind toe, known as the hallux, is positioned on the same level as the three front toes and opposes them, allowing the bird to securely grip a branch.1

While this structure is often described as anisodactyl—a common arrangement in many birds featuring three toes directed forward and one backward—the defining feature for passerines is the specific placement of the hallux on the same level as the front toes, which facilitates their characteristic perching ability.12