Optimum Fuel Loads in Migratory Birds: Distinguishing Between Time and Energy Minimization

By combining the potential flight range of fuel with different migration policies, the optimum departure fuel load for migratory birds can be calculated. We evaluate the optimum departure fuel loads associated with minimization of three different currencies: (1) overall time of migration, (2) energy cost of transport and (3) total energy coast of migration. Predicted departure loads are highest for (1), lowest for (2) and intermediate for (3). Further, currencies (1) and (3) show departure loads dependent on the fuel accumulation rate at stopovers, while (2) is not affected by variation in the rate of fuel accumulation. Furthermore, fuel loads optimized with respect to currency (3) will differ depending on the size (body mass) of the bird and the energy density of the fuel. We review ecological situations in which the various currencies may apply, and suggest how a combination of stopover decisions and observations of flight speed may be used to decide among the three cases of migration policies. Finally, we calculate that the total energy cost of migration is roughly divided between flight and stopover as 1:2. The total time of migration is similarly divided between flight and stopover as 1:7, probably with a relatively longer stopover time in larger species. Hence, we may expect strong selection pressures to optimize the fuel accumulation strategies during stopover episodes.