Why do young hoatzins have claws on their wings?

Young hoatzins possess functional claws on the first and second digits of their wings to provide them with mobility during their flightless period. Because hoatzin nests are typically built over water, these claws allow the nestlings to scramble through branches and, if they fall into the water to escape danger, to climb back up to the safety of their nest.12

While these claws were once erroneously thought to indicate an evolutionary link to the reptilian ancestors of birds, most authorities now consider them a secondary adaptation that enhances the survival of the young.1