Nutritional requirements for maintenance of body weight and fat deposition in the long-distance migratory garden warbler, Sylvia borin (Boddaert).

Intake of food, protein, fat and carbohydrates and their fecal output and the birds' weights were recorded during different feeding trials with specific nutrient reduced diets in the old-world long-distance migratory garden warbler. The birds' body weights were affected by low dietary protein as well as low dietary fat levels. Low dietary protein and fat levels were associated with significant changes in daily gross and net food intake and in the efficiency of food and nutrient utilization. Birds fed on diets with low nutrient levels for an extended length of time recovered in weight after an initial weight loss. They obviously compensated the restricted nutrient levels primarily by increasing the daily food intake and by changing the efficiency of food and nutrient utilization. Effects of restricted dietary nutrient levels on body weight and adaptation depended on the previous composition of the food. The average daily net fat intake was much higher than the average daily net protein intake, both for maintenance of a constant body weight and for successful regain of weight. The data were further discussed with respect to the role of a fruit diet in omnivorous passerine birds.