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

Environmental changes are responsible for the evolution of flexible physiol ogy and the extent of phenotypic plasticity in the regulation of birds' org an size has not been appreciated until recently. Rapid reversible physiolog ical changes during different life-history stages are virtually only known from long-distance migrants, and few studies have focused on less extreme a spects of organ flexibility. During moult, birds suffer from increased wing loading due to wing-area reductions, which may impair flight ability. A pr evious study, found that tree sparrows' escape flight (Passer montanus) is unaffected during moult, suggesting compensatory aptness. We used non-invas ive 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 th is ratio decreased. We show experimentally a novel, strategic, organ-flexib ility pattern. Unlike the general pattern, where body mass is positively co upled to pectoral-muscle size, tree sparrows responded within 7 days to red uctions in wing area by reducing body mass concurrently with an increase in pectoral-muscle size. This rapid flexibility in a non-migratory species pr obably, reflects the paramount importance and long history of flight in bir ds.