Energetics of fattening and starvation in the long-distance migratory garden warbler, Sylvia borin, during the migratory phase

Garden warblers (Sylvia borin) were subjected to starvation trials during their autumnal migratory phase in order to simulate a period of non-stop migration. Before, during and after this treatment the energy expenditure, activity, food intake and body mass of the subjects were monitored. Assimilation efficiency was constant throughout the experiments. The catabolized (during starvation) and deposited body tissue (during recovery) consisted of 73% fat. Basal metabolic rate was decreased during the starvation period and tended to a gradual increase during the recovery period. The reduced basal metabolic rate can possibly be attributed to a reduced size/function of the digestive system, which is consistent with the sub-maximal food intake immediately after resuming the supply of food to the experimental birds. The observed reductions in basal metabolic rate during starvation and activity during recovery can be viewed as adaptations contributing to a higher economization of energy supplies. The experimental birds were unable to eat large quantities of food directly after a period of starvation leading to a comparatively low, or no increase in body mass. Such a slow mass increase is in agreement with observations of migratory birds on arrival at stop-over sites.