Deep in the swamps of South America, the hoatzin (Opisthocomus hoazin) stands out as one of the stranger avian mysteries on Earth. While adult hoatzins are foliage-eating birds with uniquely enlarged crops for foregut fermentation, the chicks possess an extraordinary evolutionary feature: functional claws on their thumb and index fingers. When threatened by predators like snakes or monkeys, young hoatzins dive straight out of their branches into the swamp water below.
Once the danger passes, the chick uses its wing claws alongside its beak and feet to climb back up tangled mangrove roots and tree trunks. This bizarre trait relies on transient developmental gene expression, selectively re-activating ancestral genetic pathways that were typical of early feathered dinosaurs like Archaeopteryx. As the bird matures into adulthood, the claws drop off or become completely absorbed, and the hand bones fuse into normal wing structures.