Diurnal Variation of Metabolic Responses to Short-Term Fasting in Passerine Birds during the Postbreeding, Molting and Migratory Period

Abstract

Prior studies have shown that plasma metabolite levels differ between feeding and short-term fasted passerines. This study investigates whether the metabolic response to short-term fasting varies during the course of the daylight hours or between birds during the postbreeding, molting and migratory period. In four species of migratory passerines, six plasma metabolite levels were measured in short-term fasted and feeding birds. Compared with feeding birds, short-term fasted birds showed decreased plasma triglyceride and glucose levels, increased β-hydroxy-butyrate and free fatty acid levels, and stable uric acid levels. This indicates that short-term fasting birds use lipids from adipose tissues and from the diet, but do not spare protein and carbohydrates. Birds during the postbreeding, molting and migratory period generally showed a similar response to short-term fasting. The metabolic response to short-term fasting was modified during the course of the day. In the early morning, short-term fasted birds exhibited similar metabolite levels as overnight fasted birds. During the course of the day, β-hydroxy-butyrate levels increased progressively less during short-term fasting, probably as a consequence of restored glycogen stores and increased food-intake and lipid stores.