Since the early 1960s it has been held that migrating birds deposit and use only fat as fuel during migratory flight, with the non–fat portion of the body remaining homeostatic. Recent evidence from field studies has shown large changes in organ sizes in fuelling birds, and theory on fuel use suggests protein may be a necessary fuel during flight. However, an absence of information on the body condition of migrants before and after a long flight has hampered understanding of the dynamics of organs during sustained flight. We studied body condition in a medium–sized shorebird, the great knot (Calidris tenuirostris), before and after a flight of 5400 km from Australia to China during northward migration. Not only did these birds show the expected large reduction in fat content after migration, there was also a decrease in lean tissue mass, with significant decreases in seven organs. The reduction in functional components is reflected in a lowering of the basal metabolic rate by 46%. Recent flight models have tried to separate the ‘flexible’ part of the body from the constant portion. Our results suggest that apart from brains and lungs no organs are homeostatic during long–distance flight. Such organ reductions may be a crucial adaptation for long–distance flight in birds.

1. Quantification of the energetic needs of reproducing animals provides a basis for understanding patterns in reproduction. The doubly labelled water technique...

SUMMARY Aiming to interpret functionally the large variation in gizzard masses of red knots Calidris canutus, we experimentally studied how the digestive processing rate is influenced by the size of the gizzard. During their non-breeding season, red knots feed on hard-shelled molluscs, which they ingest whole and crush in their gizzard. In three experiments with captive birds we tested predictions of the hypothesis that gizzard size, via the rate of shell crushing and processing, constrains intake rate in red knots (against the alternative idea that external handling times constrain intake rate). Gizzard size within individual birds was manipulated by varying the hardness of the diet on offer, and was confirmed by ultrasonography. The results upheld the `shell-crushing hypothesis9 and rejected the `handling time hypothesis9. Intake rates on with-shell prey increased with gizzard size, and decreased with shell mass per prey. Intake rates on soft (without shell) prey were higher than on with-shell prey and were unaffected by gizzard size. Offering prey that were heavily shelled relative to their flesh mass led to energy intake rates that were marginally sufficient to balance the daily energy budget within the time that is naturally available in a tidal system. We predicted the optimal gizzard sizes that are required to either (1) balance energy income with energy expenditure, or (2) to maximise net daily energy intake. The gizzard mass of free-living red knots in the Wadden Sea is such that it maximises daily net energy intake in spring when fuelling for migration, while it balances energy budget throughout the remainder of the year.

SUMMARY: The ability to change organ size reversibly can be advantageous to birds that perform long migrations. During winter, red knots (Calidris canutus) feed on shellfish and carry a muscular gizzard that weighs 10% of their body mass. Gizzard size decreases when these birds eat soft foods, e.g. while breeding in the tundra. We studied the reversibility and time course of such changes using ultrasonography. Two groups of shellfish-adapted knots (N=9 and N=10) were fed alternately a hard and a soft food type. Diet switches elicited rapid reversible changes. Switches from hard to soft food induced decreases to 60% of initial gizzard mass within 8.5 days, while switches to hard food induced increases in gizzard mass to 147% within 6.2 days. A third group of knots (N=11), adapted to soft food for more than 1 year, initially had very small gizzards (25% of the mass of shellfish-adapted gizzards), but showed a similar capacity to increase gizzard size when fed shellfish. This is the first non-invasive study showing rapid digestive organ adjustments in non-domesticated birds.

Summary: Within a single year, long‐distance migrants undergo a minimum of four cycles of fuel storage and depletion because their migrations have at least one stopover. Each cycle includes an almost twofold change in body mass (m_b). Pervasive predation threats beg the question whether escape flight abilities keep up with such large changes in m_b. We derive aerodynamic predictions how pectoral muscle mass (m_pm) should change with m_b to maintain constant relative flight power. We tested these predictions with data on red knot Calidris canutus, a long‐distance migrating wader that breeds in arctic tundra and winters in temperate and tropical coastal areas. We focused on the subspecies C. c. islandica. m _pm varied with m_b in a piecewise manner. In islandica knots with m_b ≤ 148 g, the slope (1·06) was indistinguishable from the prediction (1·25). In heavy knots (m_b > 148 g) the slope was significantly lower (0·63), yielding a m_pm 0·81 times lower than predicted at pre‐departure weights (210 g). Manoeuvrability tests showed that above 160 g, knots were increasingly unable to make a 90° angle turn. This is consistent with m_pm being increasingly smaller than predicted. Relatively low m_pm enables savings on mass and hence flight costs, and savings on overall energy expenditure. We predict that reduced escape flight ability at high m_b will be compensated by behavioural strategies to minimize predation risk.