Metabolic differences between the postbreeding, moulting and migratory periods in feeding and fasting passerine birds

1. After the breeding season, migrant passerine birds moult and subsequently migrate to their winter quarters. Moult and migration involve different physiological processes (replacement of body proteins vs energy storage for endurance flights). This study investigates metabolic responses to three phases of the annual cycle (postbreeding, moulting and migratory periods) in three physiological situations (feeding during the day, overnight fasted, short-term fasted) by examining six plasma metabolite levels of three bird species. 2. In birds feeding during the day, diurnal body mass gain, triglyceride and free fatty-acid plasma levels were higher during the migratory period than during the postbreeding and moulting periods. This reflects hyperphagia and hyperlipogenesis in preparation for migratory flights. 3. No clear effects of moult were found on the metabolite levels examined. 4. Overnight fasting was generally characterized by low protein catabolism (low uric acid levels) and increased fat utilization (high free fatty-acid, glycerol and beta-hydroxybutyrate levels), compared with feeding birds. 5. During the migratory period, however, overnight fasted birds showed no elevated free fatty-acid levels, a more marked drop in triglyceride levels, unchanged glucose levels, less increased beta-hydroxy-butyrate levels and a stronger decrease in uric acid levels than birds in the postbreeding and moulting periods. This suggests that the increased fat deposition rate during the migratory period leads to increased hepatic and muscular lipid stores by the end of the day. During the migratory period, a higher utilization of these triglycerides allows a lower utilization of fatty acids from adipose tissues and a more effective sparing of protein and carbohydrates during overnight fasting than during the postbreeding period. 6. These metabolic responses were more pronounced in the Blackcap and Robin than in the Garden Warbler which starts migration when it is in the last stages of moult.