The hoatzin (Opisthocomus hoazin), often called the reptile bird, is an evolutionary anomaly native to the swamps and flooded forests of South America. While adult hoatzins are notable for their unique foregut fermentation diet—digesting leaves much like cows—their chicks possess a far more striking morphological trait. Each hoatzin hatchling is born with two functional claws protruding from the joints of its wings.
When threatened by arboreal predators such as monkeys or snakes, the wing claws serve a crucial survival function. The chick drops directly from its overhanging nest into the river below to escape. After swimming to the safety of nearby vegetation using its feet and wings, the chick uses its sharp wing claws along with its feet to haul itself back up thick tree trunks and branches into the canopy.
As the bird matures and develops full flight feathers, these claws are lost through developmental regression, leaving the adult without them. This transient juvenile feature provides a compelling glimpse into evolutionary adaptations reminiscent of ancient theropod ancestors like Archaeopteryx.