Metabolic responses to diurnal feeding patterns during the postbreeding, moulting and migratory periods in passerine birds

1. This study investigates the metabolic response (a) to the onset of feeding in the early morning, (b) to the different feeding activity patterns of four species of passerines and (c) changes in the diurnal course of the metabolic response to the different requirements of moult and migratory fattening. Six plasma metabolite levels were examined in free-living Erithacus rubecula, Sylvia atricapilla, S. borin and Ficedula hypoleuca. 2. Plasma triglyceride concentration showed a very discriminating response to feeding activity and metabolic requirements, and the following factors were shown to affect the diurnal pattern of plasma triglyceride concentration: (a) moult affected the slope of the diurnal increase in plasma triglyceride concentration; (b) the amount of food ingested (hyperphagia) affected the levels of plasma triglycerides; (c) the diurnal activity pattern (feeding pattern) affected the shape of the diurnal triglyceride pattern. 3. Plasma free fatty-acid and glycerol concentrations showed no or only slight diurnal trends, in accordance with the absence of increased levels in overnight fasted birds. Plasma uric acid levels did not show diurnal trends, despite generally lower levels at the end of the night. 4. Plasma beta-hydroxy-butyrate levels decreased very quickly in the early morning from the high levels observed in overnight fasted birds. 5. Plasma glucose levels increased during the course of the day only in birds during the migratory period, probably as a response to hyperphagia.