Fuel and fly: adaptations to endurance exercise in migrating birds

Abstract: Birds on migration alternate between consuming fuel stores during flights and accumulating fuel stores during stopovers. This thesis highlights some of the ways in which migrating birds have adapted to the different demands of fuelling and flight. Most of the time on migration is spent at stopover sites accumulating fuel stores. To minimise the total time spent on migration, birds should fuel up as fast as possible. I show that migrating birds have an exceptional energy assimilation capacity, enabling rapid accumulation of fuel stores. Migrating birds can increase their daily energy assimilation, and fuel accumulation rates, by utilizing a larger part of the day for foraging. There is also evidence for an adaptive flexibility in this digestive capacity and that digestive capacity can be built up rapidly following depletion of fuel stores due to flight. Fuel economy is crucial during long distance migratory flights. I present the first estimates of metabolic power, or rate of fuel consumption, for migratory birds performing sustained flight in a windtunnel. The way metabolic power increases with body mass in the red knot (Calidris canutus) indicate that the flight muscles are adapted for fuel efficiency in long flights with heavy fuel loads. Metabolic power curves and minimum power speeds for a thrush nightingale (Luscinia luscinia) and a teal (Anas crecca), estimated from mass loss rate, indicate that the drag of the birds bodies in flight is lower than previously thought. Fat is the main fuel for long migratory flights. I show that protein makes a significant contribution to the energy metabolism during sustained flights in the thrush nightingale. Net protein catabolism may reflect physiologically inevitable processes, may provide extra water to counteract dehydration during flight, or may reflect adaptive changes in the size of organs. Intraindividual variation in BMR, protein catabolism during flight and protein deposition during fuelling all indicate that migrants flexibly adapt their morphology and physiology to the different demands of fuelling and flight. Changes in pectoral muscle size of red knots may be an adaptation to maintain optimal flight performance when body mass varies. Maintaining heat balance in flying birds, especially at high ambient temperatures, can create problems with water balance. Red knots flying at lower ambient temperatures regulated dry heat loss and maintained water loss at a constant low level. At higher temperatures evaporative heat loss increased sharply, resulting in a net water loss. Maximum flight range in migrating birds imposed by energy and water budgets are predicted using an updated physiological computer model. Comparing the outcome of this model with experimental data indicate that the model predictions appear to be realistic but are associated with considerable uncertainties.