Jenni, L. & S. Jenni-Eiermann (1998)

    Fuel supply and metabolic constraints in migrating birds.

    Further information

    J. Avian Biol. 29: 521–528



    Energy management for endurance flight critically determines the ecological options in the life history of migrant birds. Apart from the amount of energy stores, the types of fuels used and metabolic constraints of fuel supply and oxidation determine endurance performance in long-distance migrants. The three main types of fuel (lipids, glycogen, protein) are evaluated regarding (a) costs of transport and maintenance, (b) supply to the muscles during flight, (c) adverse effects during, and (d) after, flight, and (e) refuelling. Lipids are the best fuel type in three out of these five criteria, but pose problems regarding the supply from adipose tissues to the muscles and regarding oxidation. Nevertheless, birds can maximize the contribution of energy derived from lipids as a proportion of the total energy expenditure to about 95% during migratory endurance flight. Mechanisms to do so may include enhancing fatty acid transport to the flight muscles, increasing initial fat stores and increasing maximal aerobic capacity. A very low relative contribution of energy derived from protein (RPC) is attained (around 5%), similar to long-term fasting birds. RPC values of fasting birds depend on initial fat content and are lower than in fasting mammals. This suggests that in fasting birds the absolute amount of protein catabolism is roughly proportional to metabolic rate. Mechanisms to increase the relative contribution of energy derived from fat are costly. Hence, they are expected to modify the power curve relating energy expenditure to flight speed and predictions derived thereof. It is also expected that migrants flying short non-stop distances or risking dehydration will not maximize lipid utilization to the same extent as birds flying long distances non-stop. Hence, the optimal amount and proportions of fat and protein stored and the functional organ size is expected to depend on the migration strategy, particularly on the duration of non-stop flights and the risk of dehydration.