During long-distance migration, passerine birds alternate between Eight and stopover phases. Flight phases can last one to three consecutive nights with rest during the days; stopover phases last one to three weeks. During long flight phases, such as across the Sahara desert, the substrate of catabolism is fat and protein in proportions of about 3 to 1. Comparison of migrating Garden Warblers Syliva borin before and after the desert crossing shows 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 food 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 extended Eights, without food intake, protein metabolites serve as intermediates 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 functional consequences of hypertrophy and atrophy in specific organs. Flexibility in size of the breast muscle may also be a mechanism to adjust the power output to the changing power requirement during Eight with change in fat load. Flexibility in the digestive system may save maintenance energy during Eight at the cost of restoration during stopover.
