Influence of body composition on the metabolic rate of nestling European shags (Phalacrocorax aristotelis)

During the early development of avian nestlings, their mass-specific restin g metabolic rate (RMR) changes in a biphasic pattern with the peak value of ten being much higher than that expected for an adult bird of similar body mass. In the present study we examined the possible influence of variations in the size of internal organs in "setting" the high RMR and peak metaboli c rate (PMR) during development in a large altricial species, the European shag (Phalacrocorax aristofelis). Thermoneutral RMR and cold-exposure induc ed PMR were measured in nestlings 15 days old, the age at which the highest RMR occurred during development. Body mass averaged 414 g. Mean values of RMR and PMR were 5.75 W and 9.08 W, respectively; the RMR value corresponds to approximately 250% of the expected value for an adult non-passerine bir d of similar body mass. The masses of all the organs measured (breast and l eg muscles, heart, liver, intestine, and kidney) varied isometrically with total body mass. However, large chicks had a significantly lower fractional water content than small chicks, suggesting that the former had achieved a higher level of functional maturity. In contrast to what has been suggeste d for adult birds in general, the heart and kidney masses of shag nestlings were not significantly correlated with the metabolic rates. The intestine length, in contrast, was highly and positively correlated with both the RMR and the PMR, i.e. intestine length was a better predictor of RMR and PMR t han was total body mass. In addition, liver mass was positively correlated with RMR. The results of the present study suggest that the liver in partic ular may play a key role in establishing the high, mass-specific RMR which is attained during development in bird chicks. Our results also support pre vious suggestions that early in their development, altricial chicks mainly allocate energy to the growth of 'energy-processing' organs (such as the in testine and liver) rather than to 'energy-consuming' organs.