The Juvenile Hoatzin (Opisthocomus hoazin)
It should first be noted that all birds are dinosaurs (order Saurischia, clade Theropoda), not just descendents of dinosaurs - modern genetic analysis strongly supports this cladistic organization. But given what we’re too often taught in schools, birds and dinosaurs are hard to reconcile in many peoples’ minds.
The juvenile hoatzin, however, makes it easy to see the reptilian traits that once dominated the early birds, and displays the unused genetic codes that lurk in the genome of modern avians. When they hatch, they’re equipped with lizard-like claws in front of their wings. Their use is described here, but in short, they use them to return to their nest and avoid predators. Their claws disappear by the time they leave the nest, having grown together into the metacarpals that support the wing structure.
Another fascinating trait of the hoatzins is their vegetarianism and their digestive tract. They have gut flora and fermentation similar to ruminants, which no other bird has. This is actually what leads to their being called “stink birds" - they exude a lot of stench with the fermentation process. The gut fermentation is so important to the hoatzin that the flight muscles attached to their keel are significantly reduced, to allow for more space for the stomach. They are weak flyers because of this. After a large meal, an adult hoatzin can spend up to two days doing almost nothing, allowing the leaves and greenery to have their nutrients released by their symbiotic gut flora.
Top: Attitudes of the juvenile hoatzin while climbing
Second row, left: Hoatzin nest with two eggs - Note proximity to water
Second row, right: Two hoatzin chicks preparing to dive, after appearance of threat from above
Third row, left: Hoatzin chick demonstrating strong swimming abilities
Third row, right: Hoatzin chick demonstrating poor locomotion on land
Bottom: Detail of hoatzin chick climbing, using neck, feet, and claws.
Tropical Wild Life in British Guinea, Vol 1. Curated by William Beebe, 1898.