Rapid changes in the size of different functional organ and muscle groups during refueling in a long-distance migrating shorebird

The adaptive value of size changes in different organ and muscle groups was studied in red knots (Calidris canutus islandica) in relation to their mig ration. Birds were sampled on five occasions: at arrival in Iceland in May 1994, two times during subsequent refueling, at departure toward, and on re turn from, the high arctic breeding grounds. During their 24-d stopover in May, body mass increased from 144.3 to 214.5 g. Mass gains were lowest over the first week (0.85 g/d, only fat-free tissue deposited). Over the subseq uent 10 d, average mass increased by 5.0 g/d (fat contributing 78%), and ov er the last week before takeoff, it increased by 2.0 g/d (fat contributing over 100% because of loss of lean components). There were no sex difference s in body and fat mass gains. Over the first interval, lean masses of heart , stomach, and liver increased. During the middle 10 d, sizes of leg muscle , intestine, liver, and kidneys increased. Stomach mass decreased over the same interval. In the last interval before takeoff, the stomach atrophied f urther and the intestine, leg muscles, and liver became smaller too, but pe ctoral muscles and heart increased in size. Sizes of "exercise organs" such as pectoral muscle and heart were best correlated with body mass, whereas sizes of organs used during foraging (leg muscles) and nutrient extraction (intestine, liver) were best correlated with rate of mass gain. Kidneys cha nged little before takeoff, which suggests that they are needed as much dur ing flight as during refueling.