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SUMMARY Many different physiological changes have been observed in wild waterfowl during the flightless stage of wing moult, including a loss of body mass. We aimed to determine whether captive barnacle geese (Branta leucopsis)would show the characteristic decrease in body mass during their wing moult,even though they had unlimited and unrestricted access to food. Fourteen captive geese were weighed at 1–2-week intervals for two complete years. During the flightless period of the moult, body mass decreased by approximately 25% from the pre-moult value. To understand the basis of this change, the rate of oxygen consumption was measured during daytime and nighttime at six points in the second year, and at three points (before,during and after wing moult) behavioural observations were made. Measurements of the rate of oxygen consumption showed an 80% increase above that of the nonmoulting periods of the year. We propose that metabolism was increased during moult because of the cost of feather synthesis. Although food was available, the captive birds chose not to forage and instead increased the proportion of time spent resting. It is likely that this behaviour in response to wing moult is a strategy to avoid predation in the wild. Thus, the innate nature of this behaviour has potential survival value for wild birds of this species. We conclude that the increase in metabolism led to the use of endogenous energy reserves because the birds reduced rather than increased their food intake rates, and as a result, the barnacle geese lost body mass during wing moult.

Organisms vary widely in size from microbes weighing 0.1 picograms to trees weighing thousands of megagrams, a 10 21-fold range similar to the difference in mass between an elephant and the Earth. Mass has a pervasive influence on biological processes but the effect is usually non-proportional; for example, a 10-fold increase in mass is typically accompanied by just a 4-to-7-fold increase in metabolic rate. Understanding the cause of allometric scaling has been a long-standing problem in biology. Here, we examine the evolution of metabolic allometry in animals by linking microevolutionary processes to macroevolutionary patterns. We show that the genetic correlation between mass and metabolic rate is strong and positive in insects, birds, and mammals. We then use these data to simulate the macroevolution of mass and metabolic rate, and show that the interspecific relationship between these traits in animals is consistent with evolution under persistent multivariate selection on mass and metabolic rate over long periods of time.

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