Seabirds may be particularly vulnerable to neonatal food restriction because their nestling periods tend to be long and parents may not increase foraging effort during times of prey shortage. We performed a captive study of Rhinoceros Auklet (Cerorhinca monocerata) nestlings to identify adaptations for coping with food shortages, as well as possible consequences of early diet restriction on subadult morphology. We tested effects of a ~50% caloric restriction on Rhinoceros Auklet morphological allocation and levels of the stress hormone corticosterone. Rhinoceros Auklets were reared in captivity and provisioned either ~441 kJ per day or ~227 kJ per day of high-quality forage fish until fledging (n = 13 for both treatment groups). Food-restricted Rhinoceros Auklets allocated energy heavily toward skeletal growth at the expense of mass reserves, resulting in fledglings that were proportioned very differently compared to nonrestricted birds—i.e., at 42 days of age, all birds had approximately the same wing length despite a 95 g difference in mass. Nestlings with restricted diets exhibited low plasma concentrations of corticosterone over the course of the experiment, although baseline levels were consistently higher than those of nestlings not restricted in diet. At 11 months of age, birds whose diets had been restricted were still ~50 g lighter than those whose diets had not been. The former had smaller tarsi, marginally smaller culmens, but similar-sized manus. Our results suggest Rhinoceros Auklets use both morphological allocation and adrenocortical suppression to cope with energy shortages in the nest.