Figure 5.1 Body mass changes, reflecting pre-migratory fuel deposition, in male White-crowned Sparrows Zonotrichia leucophrys gambelii caught repeatedly in autumn in California. The maximum rate recorded was from a bird which went from 26.5 g to 30.4 g in 22.5 hours, a 14.7% increase. From Morton (2002).
The pectoral muscles of birds may form more than a third of total body mass, and consist predominantly of fast oxidative glycolyic fibres, which are able to beat the wings continuously at high frequencies for hours or days on end. The flight muscles of many birds are especially adapted for utilising fatty acids for energy. These muscles are highly vascularised, with greater capillary-to-fibre ratio than other muscles (Butler & Woakes 1990), and are well supplied with mitochondria and aerobic enzymes for the oxidation of fatty acids (for review, see Ramenofsky 1990). In fact, the smallest fibres and greatest capillary densities are found in the flight muscles of birds which migrate the longest distances (Lundgren & Kiessling 1988). Fatty acids are transported from the adipose tissue to the flight muscles by the bloodstream, bound either to albumin or to lipoproteins. During long flights, power is produced almost entirely from aerobic metabolism, the respiratory and cardiovascular systems supplying the necessary fuels and oxygen, and also removing the various metabolic end products (such as carbon dioxide and heat).
An idea of the amount of fuel deposited by migratory birds can be gained from their body weights (comparing individuals at different stages of fattening; Figure 5.1), and from their 'fat scores' based on the yellowish fat that can be seen through the skin of a live bird when the feathers are blown aside. Most such studies record the fat in the furculum (wishbone), which appears as a V-shaped hollow at the base of the neck on the underside. Such scores are useful but do not bear a linear relationship to the total fat in the bird's body; nor are they comparable between species. Their value is that they can be recorded without harm from live birds, and can be used for comparative purposes within species. More detailed studies of fuel deposition have involved analyses of bird carcasses in order to find the relative proportions of fat, water and lean dry material (the latter comprising mainly body protein, feathers and skeleton) (Tables 5.2 and 5.3). Such studies have shown how the body composition of particular species changes during the course of migration, and how these changes vary between species, according to
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