Body building and concurrent mass loss: ight adaptations in tree sparrows

Environmental changes are responsible for the evolution of £exible physiology and the extent of pheno-typic plasticity in the regulation of birds ’ organ size has not been appreciated until recently. Rapid reversible physiological changes during di¡erent life-history stages are virtually only known from long-distance migrants, and few studies have focused on less extreme aspects of organ £exibility. During moult, birds su¡er from increased wing loading due to wing-area reductions, which may impair £ight ability. A previous study found that tree sparrows ’ escape £ight (Passer montanus) is una¡ected during moult, suggesting compensatory aptness. We used non-invasive techniques to study physiological adaptations to increased wing loading in tree sparrows. As wing area was reduced during natural moult the ratio of pectoral-muscle size to body mass increased. When moult was completed this ratio decreased. We show experimentally a novel, strategic, organ-£exibility pattern. Unlike the general pattern, where body mass is positively coupled to pectoral-muscle size, tree sparrows responded within 7 days to reductions in wing area by reducing body mass concurrently with an increase in pectoral-muscle size. This rapid £exibility in a non-migratory species probably re£ects the paramount importance and long history of £ight in birds