Avian pectoral muscle size rapidly tracks body mass changes during flight,fasting and fuelling

We used ultrasonic imaging to monitor short-term changes in the pectoral mu scle size of captive red knots Calidris canutus. Pectoral muscle thickness changed rapidly and consistently in parallel with body mass changes caused by flight, fasting;and fuelling. Four knots hew repeatedly for 10h periods in a wind tunnel. Over this period, pectoral muscle thickness decreased in parallel with the decrease in body mass. The change in pectoral muscle thic kness during flight wats indistinguishable from that during periods of natu ral and experimental fasting and fuelling, The body-mass-related variation in pectoral muscle thickness between and within individuals was not related to the amount of Right, indicating that changes in avian muscle do not req uire power-training as in mammals. Our study suggests that it is possible f or birds to consume and replace their flight muscles on a time scale short enough to allow these muscles to be used as part of the energy supply for m igratory flight. The adaptive significance of the changes in pectoral muscl e mass cannot be explained by reproductive needs since our knots were in th e early winter phase of their annual cycle. Instead, pectoral muscle mass c hanges may reflect (i) the breakdown of protein during heavy exercise and i ts subsequent restoration, (ii) the regulation of flight capacity to mainta in optimal flight performance when body mass varies, or (iii) the need for a particular protein:fat ratio in winter survival stores.