Compensatory bodily changes during moult in Tree Sparrows Passer montanus in Italy

Abstract

To cope with fluctuating environments animals have evolved reversible phenotypic flexi-bility. Some birds demonstrate this phenomenon by changing mass and flight muscle ac-cording to changes in wing loading. During moult, birds suffer from reduced wing area because feathers are shed and replaced, resulting in a wing loading increase. Moult is rather well studied in birds, but the perspective of phenotypic flexibility has been ne-glected. Therefore, we tested predictions generated from experimental studies by collect-ing information about body mass, flight muscle size and fat stores from an Italian popula-tion of Tree Sparrows (Passer montanus) to investigate if they compensate physiologi-cally for the wing area reductions they suffer from during moult. Our results did not cor-roborate predictions based on experimental studies; that is, the Tree Sparrows did not re-duce body mass and increase in flight muscle size as a response to wing area reductions during midmoult. Instead, body mass increased throughout moult, flight muscle size did not change, and fat stores decreased as moult progressed. To further investigate compen-satory changes, we analysed bodily differences in midmoult between birds differing in moult gap size. Again, contrary to predictions from experimental studies, birds having larger moult gaps were found to have higher body mass. These birds were also found to keep the ratio between flight muscle size and body mass constant over the day whereas birds with small moult gaps reduced this ratio over the day. Birds with large moult gaps were also found to store less fat than birds with small gaps. Physiological constraints may help to explain these results and underlying reasons for the observed variation in bodily regulation in birds are discussed.