BAUCHINGER, U; BIEBACH , H; University of Munich (LMU), Germany; Max-Planck-Institute for Ornithology: Phenotypic flexibility of organs and its releationship to body mass changes during migration

Long-distance migration across ecological barriers necessitates substantial internal resources. Deposition of those resources can result in a doubling of body mass. Fat is the dominating fuel for flight, but protein from tissue may amounts for up to 1/3 of the body mass changes. Within a few days of sustained flight, body mass may drop to levels below the initial value. We sampled garden warblers Sylvia borin before spring migration in Tanzania, during stopover in Ethiopia, and post-flight in Egypt and quantified the mass of 13 organs. Flying from Ethiopia across the Sahara to Egypt resulted in a loss of lean dry organ mass of 26% in flight muscle, 14% in leg muscle, 24% in heart, 50% in the digestive tract, 57% in liver, and 40% in kidney. A current hypothesis about phenotypic flexibility of organs assumes a positive relationship between organ mass and body mass. Originally proposed for the flight muscle, it predicts size adaptation of the muscle to the changing power requirements due to changing body mass. ANCOVA (corrected for body mass) revealed that two organs, leg muscle and small intestine were significantly related to body mass, but only the changes in the leg muscle could be explained by its relationship to body mass. Proventriculus, gizzard, colon, pancreas, gall bladder, spleen, kidney and testis were not related to body mass. Flight muscle, heart and liver showed a significant interaction between body mass and sampling site and thus were not uniformly related to body mass during the migratory period. Only birds sampled in Egypt showed a significant relationship between flight muscle and body mass. In summary, we showed that different functional hypothesis for a protein metabolism during migration, seem to be appropriate for different organs at different stages of migration.