En route: Energy metabolism of bird migration

During migration birds will go repeatedly through two behavioural and physiological stages: fuelling and the endurance flight. Fuelling is the anabolic phase, during which the birds replenish their fuel stores for flight; flight is the catabolic phase, during which the fuel stores are broken down. Our projects investigated how the energy metabolism adapts to these demands.

Aims

To meet the high energy demands of flight, the birds have to rely exclusively on their body energy stores. Our study investigates possible metabolic adaptations which a) supply the muscles of flying birds with sufficient energy and b) enable feeding birds to increase their body mass quickly and efficiently. The study also investigated the importance and quality of stop-over areas.

Approach

We measured plasma metabolites of the fat-, protein- and carbohydrate-metabolism in European passerines during their autumn migration. To investigate the anabolic phase of body mass increase, the birds were sampled during their stay in a resting area. The catabolic flight metabolism was investigated in birds caught out of their autumnal migratory flight on an Alpine pass in Switzerland. The plasma metabolite concentration indicates in which physiological state a bird is and which type of energy it metabolises. Additionally to the field studies the flight metabolism was investigated under controlled, experimental conditions. This was done with homing pigeons and - in collaboration with Swedish and Dutch colleagues – in red knots flying up to 10 h in a wind-tunnel.

Results

Wedemonstrated that migrating birds have special physiological adaptations which explain why a bird can accomplish such an endurance performance while fasting. The organism maximizes the use of triglycerides. Fat has the advantage that its energy density is high, hence the transport costs are low. Thanks to a special pathway used especially by long-distance migrating birds, the metabolites of fat, the free fatty acids, are transported in very high quantities to the breast muscles. Only a small amount (about 5-10%) of energy is derived from protein. This saves transport costs because protein is stored with a high amount of water and therefore, relative to its energy density, is a “heavy” fuel. Birds also decrease their flight mass by reducing their organs and the breast muscles.

Project management

Susi Jenni-Eiermann, Lukas Jenni

Partners

Dr. Ake Lindström und Anders Kvist, Universität Lund, Sweden
Prof. Dr. Theunis Piersma, Netherlands Institute for Sea Research, Texel, The Netherlands
Prof. Dr. Hubert Schwabl, Washington State University Pullman
Dr. Fernando Spina, Istituto Nazionale per la Fauna Selvatica, Bologna, Italy.