Metabolic responses to flight and fasting in night-migrating passerines

1.	Small passerine migrants achieve endurance flight while fasting, together with one of the highest mass-specific energy rates. Metabolic responses to flight and fasting were examined in three species of free-living migrants (Sylvia borin, Ficedula hypoleuca, Erithacus rubecula) by measuring plasma concentrations of glucose, uric acid, triglycerides, glycerol, free fatty acids (FFA), and -hydroxybutyrate (-OHB) in three main physiological situations (feeding, overnight fasting, nocturnal flight) and while changing between these situations.
2.	Overnight-fasted birds showed low triglyceride and uric acid levels. Contrary to mammals, FFA and glycerol levels were not increased in agreement with published data on birds. The transition from feeding to fasting (post-feeding) was distinguished by a temporary rise in FFA and a drop in glucose levels.
3.	Birds utilize fat during migratory flight, indicated by high levels of FFA, glycerol, and -OHB. For the first time, high triglyceride levels were found in an exercising vertebrate. The use of protein during flight was demonstrated by high uric acid levels.
4.	Birds kept inactive after flight showed a more pronounced reduction of the fat and protein utilization and post-exercise ketosis than naturally landed birds.
5.	Differences among the three species in the metabolic pattern suggest that the garden warbler shows the greatest metabolic adaption to endurance flight, having the highest levels of fat metabolites and the highest body fat reserves.