The body mass cycle of the migratory garden warbler (Sylvia borin) is associated with changes of basal plasma metabolite levels

Garden warblers show pronounced seasonal changes in body mass in relation to migration. In order to reveal if fattening and defattening is associated with changes in plasma levels of metabolic key factors, body mass, food intake, molt, visible fat, and plasma metabolite, and electrolyte levels were measured under constant indoor conditions for the non-reproductive period (September to May). During the phase of high body mass and high fat loads, the plasma levels of glucose, triglycerides, cholesterol and free fatty acids were higher, sodium concentration and glucose tolerance lower than in the phase of low body mass and molt. Urea, uric acid, P-hydroxybutyrate and potassium levels showed no or only small variations throughout the year with no significant relationship to the body mass cycle. In preparation for migration a metabolic change in preferring fat as substrate for energy metabolism is assumed. This perhaps results in constraints as predicted by the glucose fatty-acid cycle, which appear to be similar to those in mammalian obesity. (C) 1998 Elsevier Science Inc. All rights reserved.