Phenotypic organ flexibility in Garden Warblers Sylvia borin during long-distance migration

During long-distance migration, passerine birds alternate between Eight and stopover phases. Flight phases can last one to three consecutive nights wi th rest during the days; stopover phases last one to three weeks. During lo ng flight phases, such as across the Sahara desert, the substrate of catabo lism is fat and protein in proportions of about 3 to 1. Comparison of migra ting Garden Warblers Syliva borin before and after the desert crossing show s that the protein originates primarily from the muscles of the breast and leg and From the digestive tract. Breast and leg muscles are reduced by 19% , the digestive tract by 39% in mass. A simulation of the flight phase by f ood deprivation over 48 h results in the same extent of organ reduction in the digestive tract but not in the breast and leg muscles, which indicates that different mechanisms are involved. It is hypothesised that during exte nded Eights, without food intake, protein metabolites serve as intermediate s in the citric cycle for the oxidation of fat. Loss of protein metabolites is at the expense of tissue proteins from the above-mentioned organs. This hypothesis does not, however, exclude other hypotheses about the functiona l consequences of hypertrophy and atrophy in specific organs. Flexibility i n size of the breast muscle may also be a mechanism to adjust the power out put to the changing power requirement during Eight with change in fat load. Flexibility in the digestive system may save maintenance energy during Eig ht at the cost of restoration during stopover.