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Effects of variable energy intake on chick growth, organ development, and gut-passage time in seabirds were investigated by raising Elegant Tern (Sterna elegans) chicks in the laboratory on three diets involving a major prey species, northern anchovy (Engraulis mordax), and a minor prey species, topsmelt (Atherinops affinis). Anchovies are ∼25% higher in energy content than topsmelt and represent a more abundant but highly fluctuating prey resource. Chicks were raised from age 15 to age 35 days posthatch on one of three diets (n = 8): (1) high-energy anchovy at the average daily intake rate, (2) low-energy topsmelt at the same intake rate, and (3) high-energy topsmelt (i.e. at a biomass equal in energy content to the anchovy diet). Chicks raised on the high-energy anchovy diet had a significantly faster gain of body mass and wing length. All three groups, however, had attained a statistically similar wing length at 35 days posthatch and did not differ in lean dry mass or pectoral muscle mass, indicating that energy was allocated preferentially toward wing development and flight capacity. Most digestive organs were heavier and gut-passage rates faster for chicks fed topsmelt. Bill dimensions and tarsus length showed little difference among chicks on the three diets. Lipid content of chicks that were fed anchovy was twice that of those fed low-energy topsmelt and one-third higher than that of chicks on the high-energy topsmelt diet. Our results indicate that Elegant Terns have evolved certain capacities for growth and digestion in response to variable prey quality that may increase the probability of chick survival. Nevertheless, if Elegant Tern chicks were switched to a lower quality, primarily topsmelt diet, they would be underweight at fledging with heavier digestive organs, lower lipid reserves, and faster gut-passage times, which would place added provisioning demands on the parents. Under that scenario, the prospects for survival and long-term reproductive success might diminish for such an Elegant Tern colony.

Energy Allocation and Postnatal Growth in Captive Elegant Tern (Sterna Elegans) Chicks: Responses to High-Versus Low-Energy Diets