 The development of the remiges ( and alulae ) of nestling hoatzins is much delayed compared to the development of these feathers in other young birds , presumably because young hoatzins are equipped with claws on their first two digits . They use these small rounded hooks to grasp branches when clambering about in trees , and feathering on these digits would presumably interfere with that functionality . Most youngsters shed their claws sometime between their 70th and 100th day of life , but some retain them — though callused @-@ over and unusable — into adulthood . 
