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Fuel Supply and Metabolic Constraints in Migrating Birds

Lukas Jenni and Susanne Jenni-Eiermann
Journal of Avian Biology
Vol. 29, No. 4, Optimal Migration (Dec., 1998), pp. 521-528
Published by: Wiley on behalf of Nordic Society Oikos
DOI: 10.2307/3677171
Stable URL: http://www.jstor.org/stable/3677171
Page Count: 8
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Fuel Supply and Metabolic Constraints in Migrating Birds
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Abstract

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 fuel used and metabolic constraints 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 with respect to three 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 aerobic capacity. A very low relative contribution of energy derived from protein (RPC) is attained by migrating birds (around 5%), similar to that of 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 from it. 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 functional organ sizes are expected to depend on the migration strategy, particularly on the duration of non-stop flights and the risk of dehydration.

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