Why do parent procellariiform birds leave chicks alone before the chicks can fly?

Parent procellariiform birds desert their chicks before they are capable of flight to retire to sea to molt.1

This desertion initiates a necessary period of fasting and exercise for the chick. During this time, the young bird—which is typically well-feathered and heavier than an adult—must thin down and strengthen its wings to become airborne. This transition is critical, as the fledgling must eventually depart the nest to head to sea on its own, guided by innate impulses rather than parental instruction.1