Energetic Savings by Organ Adjustment During Long Migratory Flights in Garden Warblers (Sylvia borin)

During the migratory period, birds are confronted with many physiological challenges that may change dramatically in short succession. Flight phases with voluntary anorexia and simultaneous high metabolic turnover for flight are interrupted by stopover phases with intense foraging behaviour and high intake and processing of food. Functionally, the digestive system plays the important role during stopover and becomes irrelevant during flight. On the other hand, exercise organs have an inverse role, being highly active during flight but much less so during stopover. In the course of migration, these temporary functional differences are reflected in phenotypic organ changes (Piersma and Lindstrom 1997; Biebach 1998; Mc Williams and Karasov 2001). Several hypotheses have been put forward to explain the costs and benefits of this functional and structural flexibility, some of which are not mutually exclusive (Bauchinger and Biebach 1998). Here, we present data on structural changes in the digestive system, the leg muscles, the liver and the exercise organs — such as the flight and heart muscles — of garden warblers (Sylvia borin) during the long flight across the Sahara desert within one season during spring 1998. The benefits of these structural adjustments are estimated quantitatively with respect to the energetic savings during flight.