COMPOSITION OF FUEL STORES AND DIGESTIVE LIMITATIONS TO FUEL DEPOSITION RATE IN THE LONG-DISTANCE MIGRATORY THRUSH NIGHTINGALE, LUSCINIA-LUSCINIA

During their autumn migratory phase, thrush nightingales (Luscinia lus cinia) previously starved for 2 d were allowed to refuel under three d ifferent ambient temperature conditions (-7 degrees, 7 degrees, and 22 degrees C). During the refueling period, as well as during the preced ing control and starvation periods, food intake, body mass, and feces production were monitored. In addition, daily energy expenditure was m easured during the refueling period. The compilation of the energy bal ance during the refueling period revealed an energy density of the dep osited tissue of 33.6 kJ g(-1). Assuming that the deposited tissue con sists of fat and protein exclusively, with energy densities of 39.6 an d 5.5 kJ g(-1) wet mass, respectively, we estimated the deposited tiss ue to consist of 82% fat and 18% wet protein (6% dry protein and 12% w ater). Nitrogen balances during control, starvation, and refueling pha ses and during a period of prolonged and complete starvation indicated that 5% of the nutrient stores consisted of dry protein. Our results support recent findings that nutrient stores for migration often conta in protein in addition to fat and consequently are 15%-25% less energy rich than pure fat stores. These proteins might be stored as muscle o r other functional tissue and may be required to support the extra mas s of the stores and/or reflect an incapacity of the metabolic machiner y to catabolize fat exclusively Fuel deposition rate was positively re lated with ambient temperature, whereas food intake rate was unaffecte d by temperature. These results indicate that the rate of fuel deposit ion is limited by a ceiling in food intake rate; when this ceiling is reached, fuel deposition rate is negatively affected by daily energy e xpenditure rate. To a certain extent, the ceiling in food intake rate varies depending on feeding conditions over the previous days. These v ariations in food intake capacity probably reflect the building and br eakdown of gut tissues and/or gut enzyme systems and might be insensib le and not evolutionary adaptive. Significant energetic costs, however , are probably associated with the maintenance of gut tissues. It is t herefore feasible that changes in digestive capacity are regulated and are directed at energy economization.