Migration normally requires body reserves (mostly fat) accumulated at appropriate times of year. Depending mainly on the terrain to be crossed, birds migrate on a small fuel reserve/short flight system or on a large fuel reserve/long flight system, the latter being necessary over seas, deserts or other inhospitable areas where refuelling is not possible. Birds may vary their migration mode at different stages of their journey, depending on feeding opportunities in different parts of the route. The more fuel a bird carries, the more energy the bird uses in transporting it, which increases the energy cost per unit distance flown. Extra fuel also reduces the bird's agility in evading attacks by predators. These considerations should favour a migration strategy of short flights, frequent fuelling and low fuel loads wherever possible, with the alternative of long flights, infrequent fuelling and heavy fuel loads resorted to only when necessary.

The predominant fuel is fat which has five main advantages: (1) it provides the highest concentration of metabolic energy per unit weight; (2) it can be stored dry without accompanying water or protein; (3) it can be metabolised (by the citric acid cycle) more efficiently than protein or carbohydrate; (4) it can be oxidised efficiently and completely by most body tissues, including the all-important flight muscles; (5) muscle fibres relying on fatty acids can work for long periods without tiring. However, carbohydrate and protein are also used as fuel on migratory flights, and metabolism of fat is accompanied by metabolism of protein, which yields intermediates for the citric acid cycle, and other important metabolites. The ratio of fat to protein accumulated at migration times varies greatly between species, and even between populations of the same species, and between outward and return journeys.

In small passerine migrants, the composition of the diet seems to influence to some extent the composition of the fuel stores laid down. These stores in turn influence the relative composition of the fuel types used during migration, and (via their energy density) the flight range. The composition of body stores, notably the ratio of fat to protein, may also vary according to the nature of the journey and the terrain to be crossed, and the subsequent needs of the bird, whether for breeding or survival.

Fuel deposition results in rates of daily weight gain up to about 10% (occasionally 13% or more) of lean body mass, with slower rates in larger species, and great variation among individuals. Among passerines and shorebirds, short-distance migrants usually increase in weight by 10-30% before departure, and longdistance migrants by 70-100%, effectively doubling their body mass. Some species may accumulate no obvious reserves for migration, but travel for only parts of each day, losing weight but replenishing it on a day-to-day basis. In some sea-birds, the young accumulate fat stores while in the nest, and embark on a long migration independently of their parents soon after fledging.

Rapid weight gain for migration mainly results from increased food consumption (feeding time), but in some species also from dietary change and improved digestive efficiency (involving increase in the size of the digestive tract). Rates of weight gain may be environmentally or metabolically limited. Some migrants increase muscle and heart mass before departure, and reduce the size of other internal organs (digestive tract and liver). On arrival, the digestive organs are rebuilt before normal food assimilation can occur again. These changes in body composition are much more marked in species making long-distance non-stop flights lasting several days than in short-distance migrants that can feed and drink every day during their journeys.

Many species typically arrive on spring breeding areas with a surplus of body reserves, including both protein and fat. In some species, notably arctic-nesting geese, the reserve is used for both egg production and incubation, at a time when little food is available locally. Body reserves imported to breeding areas may thus have been accumulated on wintering and migration areas at lower latitudes. For these reasons, the various seasonal changes in the body mass and composition of migratory birds are often rapid and substantial. Nevertheless, populations vary in their level of dependence on internal body reserves for breeding.

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