Effects of diet on growth-related patterns of energy and macronutrient assimilation efficiency in a semi-precocial bird, the Gull-billed Tern Gelochelidon nilotica

Despite the extensive literature on energy assimilation efficiency in birds, only a few studies have dealt specifically with offspring, and to our knowledge there is no information on macronutrient assimilation efficiencies during the growth period of chicks. We studied growth-related energy and macronutrient assimilation efficiencies of semi-precocial chicks of Gull-billed Terns Gelochelidon nilotica as a function of their diet. We experimentally evaluated several digestive parameters in chicks fed different diets - fish vs. insects - when 10, 15 and 22 days old. Gross daily energy intake was similar in the two groups throughout the growth period, but fish-fed chicks showed consistently higher energy and macronutrient assimilation efficiencies than chicks fed insects. Energy and macronutrient assimilation efficiencies varied with age, peaking at 15 days old. The only digestive parameter that did not decrease during the last phase of growth was the lipid assimilation efficiency. Fish-fed chicks grew significantly faster than those fed on insects, which showed delayed growth. We suggest that semi-precocial chicks could modulate energy and macronutrient assimilation efficiency according to their growth requirements and that such digestive parameters can help us to understand differences in chick growth rates and breeding success in a natural environment.