ENERGETICS OF FATTENING AND STARVATION IN THE LONG-DISTANCE MIGRATORYGARDEN WARBLER, SYLVIA BORIN, DURING THE MIGRATORY PHASE

Garden warblers (Sylvia borin) were subjected to starvation trials dur ing their autumnal migratory phase in order to simulate a period of no n-stop migration. Before, during and after this treatment the energy e xpenditure, activity, food intake and body mass of the subjects were m onitored. Assimilation efficiency was constant throughout the experime nts. The catabolized (during starvation) and deposited body tissue (du ring recovery) consisted of 73% fat. Basal metabolic rate was decrease d 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 resum ing the supply of food to the experimental birds. The observed reducti ons in basal metabolic rate during starvation and activity during reco very can be viewed as adaptations contributing to a higher economizati on 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 incr ease is in agreement with observations of migratory birds on arrival a t stop-over sites.