Long-distance migrants as a model system of structural and physiological plasticity

Several migratory bird species show active cycles of hypertrophy and atrophy of nutritional organs and selected muscle groups. We develop a model for nutritional organs to identify the conditions under which such a pattern of structural change is optimal. Migrants may choose the size of two components of lean mass: a constant component that they carry throughout the entire migration period and a component just deposited during stopover. The benefit of depositing and subsequently discarding additional tissues in nutritional organs during stopover is an increased fuelling rate and a decrease in flight costs; the cost of hypertrophy and atrophy is a time cost. The predicted pattern depends strongly on the lean mass with which the migrants enter the migratory period. Time-selected migrants flying between widely separated stopover sites should show the highest degree of structural plasticity if they commence migration with a lean mass below maximum. If stopovers are possible everywhere along the migratory route, cycles of hypertrophy and atrophy are still possible but less likely. We also discuss the consequence of using different currencies in the optimization procedure. Only if duration of migration is a component of fitness can structural changes be expected. If the energy cost of transport is minimized, there is no unique prediction about active changes in lean mass. For short migration distances, it may be optimal to burn the lean mass deposited at the start of the stopover as fuel instead of discarding it before departure.