Why do procellariiform chicks become very fat before leaving the nest?

Procellariiform chicks become very fat before leaving the nest because they undergo a period of desertion by their parents, who depart to molt at sea. This results in a necessary starvation period—lasting from a week in smaller species to considerably longer in larger ones—before the fledgling is capable of flight and heads out to sea.1

During the later stages of their long fledgling period, these chicks are fed by parents who forage far at sea and return with large crop loads of food. If both parents return simultaneously, a nestling may ingest food equal to its own weight in a single meal, allowing it to become heavier and fatter than an adult. This accumulated fat serves as a vital energy reserve that sustains the chick during the post-desertion fasting phase, providing the time needed for the young bird to thin down and exercise its wings before it is ready to take its first flight.1