All creatures are fatter in migrating. (Aristotle, writing 2300 years ago.)
Some of those birds are very succulent and sanguine, and so may have their provisions laid up in their very bodies for the journey. (Written by 'a person of learning and piety', 1703.)
At times of migration, many birds put on extra body fat and other reserves for use as fuel during the journey. Typically, they divide their migration into periods of flight, during which reserves are depleted, and stopovers, when reserves can be replenished by feeding. Species that migrate over favourable terrain tend to migrate in short flights, each lasting up to several hours, broken by periods of rest and foraging, when they can replace the relatively small amounts of fuel used on each flight. Given suitable weather, migratory flight can, in theory, occur for part of every day until the journey is completed. However, birds that migrate over large inhospitable areas have to sustain much longer fasts during flights of up to several days, which are preceded by several days of feeding during which much larger reserves are accumulated. The extreme is shown by landbirds that cross large stretches of ocean, requiring up to several days and nights of non-stop flight. Passerines typically take 1-3 weeks to accumulate the fuel reserves necessary for such long journeys, and before departure some may have doubled their normal weights.
The alternating activities of fuelling and flight put different demands on the physiology of migrating birds. During fuelling, the bird should be an efficient eating machine, with a large digestive system, able to process larger than normal quantities of food rapidly for conversion to stored energy. But during flight, it must be an efficient exercise machine, with large muscles, well-functioning heart and circulatory system. It should carry enough energy-rich fuel for the journey, but a minimum of other body structures that merely add unwanted weight. One of the most interesting discoveries of recent years is that some long-distance bird migrants can drastically change not only their physiology, but also their internal body structure over periods of a few days, as they switch from fuelling mode to flight mode and back again. All muscles and body organs are energetically costly to maintain, so an ability to change their relative sizes rapidly, according to the needs of the time, can be regarded as an important adaptation, not only for migration but also for other events in the annual cycle. It enables disparate activities to be performed more efficiently than would be possible on fixed metabolic and body structures (Piersma & Lindström 1997). So-called phenotypic flexibility is apparent in other animals too, but migratory birds show some of the most extreme examples of large-scale rapid changes in body weight and structure.
In addition to fuel deposition, preparation for migration in many birds involves enlargement of the breast muscles, heart and blood vessels, and shrinkage of other organs less important in migratory flight (Piersma et al. 1999). It also involves the activation of enzyme systems for the storage and rapid mobilisation of fat, an increase in the erythrocyte (haematocrit) content of the blood to enhance oxygen transport during long flights (Thapliyal et al. 1982, Jenni-Eiermann & Jenni 1991), and the modification of different aspects of behaviour, including the diurnal rhythm of activity to permit nocturnal flights in some otherwise diurnal species (Chapter 4). This chapter is concerned with the types and amounts of fuel accumulated by birds, with the process of fuelling and other changes in body composition, and with how these features vary with the types of journeys undertaken. The full cost of migration consists of the energy needed to fuel the flight plus the energy needed to maintain the bird over the whole migration period, including stopovers. This latter cost is increased by any weather-induced delays.
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