Fat and body condition in migrating Redwings Turdus iliacus

During the night of 29-30 October 1995, over 600 Redwings Turdus iliacus di ed as a result of flying into the lighthouse at Bardsey, Gwynedd, North Wal es. These migrating birds were used to investigate Eat levels in relation t o age, sex, biometrics and pectoral muscle mass. Wing length was the best s ingle linear measure of size and mean wing length of males was 2.5 mm great er than that of females. Body mass of the casualties declined during the ni ght and the mean body mass of birds arriving towards the end of the night w as 1.5 g lower than that of the first arrivals. Fat deposits at different b ody sites were significantly correlated with each other and with body mass, and, by extrapolation, the mass of intra-abdominal far remaining would be significant when other fat deposits have been depleted. Fat in the tracheal pit (the claviculo-coracoid fat body) demonstrated the brst correlation wi th body mass and was linearly correlated with visual fat scores. Fat was al so present in the pectoral muscle but did not make a significant contributi on to overall body mass. Two-thirds of the variation in body mass was accou nted for by wing length, thr mass of claviculo-coracoid Eat and the lean-dr y mass of pectoral muscle. Claviculo-coracoid fat and lean pectoral-muscle mass contributed independently to overall body mass. These data support the view that increase in fat in relation to migration is accompanied by an in crease in protein or lean muscle mass, but suggest that these are controlle d independently.