How are the toes arranged on the feet of most piciform birds?

Most birds in the order Piciformes possess a characteristic zygodactylous (yoke-toed) foot arrangement. In this configuration, the toes are arranged in pairs, with the outer toe (toe IV) and the hind toe (toe I, or hallux) both pointing toward the rear.12

While this structure was historically interpreted as an adaptation specifically for climbing or perching on vertical tree trunks, recent research suggests it may represent a more generalized perching foot. It is important to note that variations exist within the order; for instance, some woodpecker species have lost the hind toe entirely, while others may spread their toes laterally and anteriorly while climbing.1