Procellariiform seabirds accumulate large amounts of lipid during the nestling period. Chick obesity functions as a buffer against irregular attentiveness by parents delivering meals. Surprisingly the assimilation mechanism necessary to attain obesity remains unclear. This study presents the first measurements of assimilation efficiency in relation to age and diet throughout the long nestling period of a Procellariidae. To determine the assimilation efficiency and diet effects on growth, Shorttailed Shearwater Puffinus tenuirostris chicks were fed in captivity on different diets of marine organisms (krill Euphausia superba, squid Nototodarus sloani and fish Hyperlophus vittatus) from age 10 to 85 days. Assimilation efficiency was highest (mean 93.7%) among young chicks (10–29 days old) then declined, becoming stable (mean 80.9%) among older (70–85 days old) pre-fledging chicks. It was noted that the krill-fed chicks showed consistently higher assimilation efficiencies, not only when young (mean 95.4%) but also when older (mean 84.8%), than the squid-fed or fish-fed chicks (means 92.4–93.3% when young and 78.4–79.5% when older). The krill diet contributed to better growth in body mass (close to that of free-living chicks at the source colony), while the squid- and fish-fed chicks grew slowly. The dietary effects on feather growth and molting did not parallel those on body mass increments. Squid-fed chicks had similar feather growth rates and molt progress to those of krillfed chicks, whereas fish-fed chicks often vomited their food when given sizable quantities of fish and had slower feather development and molting. These findings suggest that chicks may need parental digestive fluids in order to facilitate their digestion of larger amounts of fish, whereas they were competent to digest krill and squid on their own. When the diet of older (over 85 days of age) fish-fed chicks was changed to krill they stopped regurgitating food, gained weight and began rapid molting. Likewise, squid-fed chicks gained mass at a faster rate when they were switched to a diet of krill. The experimental Short-tailed Shearwater chicks were found to have a high absorption capability during the early nestling stage allowing for considerable lipid accumulation and resulting in obesity. This obesity facilitates the fasting necessary while their parents commute to distant nutritive waters. The shearwaters have a highly adaptive potential to assimilate krill, which are abundant organisms with a large biomass in the higher latitudes that shearwaters prefer to inhabit for foraging throughout the year. Even the krill-fed chicks, the best among the three experimental dietary groups, only attained similar body masses to free-living chicks despite being given 1.9 times as heavy meals as shearwater parents provided to their free-living chicks. This evidence indicates that in order for free-living Short-tailed Shearwater chicks to grow to obesity, they depend on a far higher energy intake than that available in their diet alone. The specific oil, accumulated in the shearwater parents' proventriculus, and delivered to the chick, is an important contribution. In other words, digestible prey, such as krill, and parental stomach oil, are both essential to insure obesity for Short-tailed Shearwater chicks.