Plasma Metabolite Levels Predict Individual Body-Mass Changes in a Small Long-Distance Migrant, the Garden Warbler

Change in body mass is an important measurement in many studies addressing changes in energy stores or condition. Usually, change in body mass is measured in birds caught twice, but this has a number of drawbacks (e.g. low number of retraps, retraps not representative of all first captures, adverse effects of first capture on body-mass development). Therefore, we investigated whether plasma metabolites correlate with body-mass change, and which metabolites could be used to predict body-mass change in birds caught once. In an experiment, 20 Garden Warblers (Sylvia borin) were given different amounts of food to induce stable, increasing, and decreasing body masses. Most of the eight plasma metabolites we examined were significantly correlated with the change in body mass between early morning and midday, the time of blood sampling, but not with body mass or various measures of activity. Metabolites that are known to characterize resorption were elevated in birds gaining body mass and metabolites characteristic of fasting were elevated in birds losing body mass. Triglycerides and ²-hydroxy-butyrate together explained 61% of the variation in body-mass change (triglycerides alone 44% and ²-hydroxy-butyrate alone 51%). These metabolites may be used to predict body-mass change in birds caught once, provided that the reliability and sensitivity of this method are checked in field tests.