Efficiency of food utilization during fat deposition in the long-distance migratory garden warbler, Sylvia borin.

1 Intake of food, fat, protein and carbohydrates and their fecal output were recorded during the annual body weight cycle of the garden warbler, and old-world longdistance migratory bird species, and the efficiencies of food and nutrient utilization, defined as the ratio (intake-fecal output)/intake, were calculated. 2. Gross food intake and food and nutrient utilization differed significantly between different phases of the birds' body weight cycle. 3. During premigratory fattening, both in "autumn" and "spring", food intake and utilization of fat, protein and carbohydrates were significantly higher than during the low body weight prefattening periods. 4. The increase in efficiency of nutrient utilization accounted for about 1/3 and the increase in gross food intake about 2/3 of all surplus energy for hyperlipogenesis in the premigratory periods of the garden warbler. 5. The seasonal changes in efficiency of food and nutrient utilization seem to be driven by a circannual timing mechanism.