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Note. One kilojoule (kJ) is equivalent to 0.239 kilocalaries, and a kilocalorie is popularly called a calorie in human dieting.

Modified from Jenni & Jenni-Eiermann (1998).

expended from wet mass)

Note. One kilojoule (kJ) is equivalent to 0.239 kilocalaries, and a kilocalorie is popularly called a calorie in human dieting.

Modified from Jenni & Jenni-Eiermann (1998).

stored without water or protein, it can also be digested efficiently with less loss of heat and no effect on body glucose. The main recognised drawback of fat is that its metabolism requires the breakdown of small amounts of protein to provide enzymes for the chemical processes involved (the citric acid cycle). In addition, while most tissues in the body can oxidise fatty acids to release energy, some tissues cannot, and rely instead on carbohydrate or ketone bodies (a reduced form of fatty acids) for energy. Such tissues include the brain and nervous system, red and white blood cells and the kidney medulla.

Fat is laid down as adipose tissue (called fat bodies) in various parts of the bird's body, especially under the skin, and in well-defined deposits within the wishbone (tracheal pit) and around the gut. At least 15 distinct fat depots have been described in passerines (King & Farner 1965). Just before departure, the subcutaneous fat layer in some long-distance passerine migrants can be so extensive that most of the body appears to be clad in a thick layer of pale-yellow fat, only the central part of the breast muscle remaining uncovered. This subcutaneous fat is relatively soft, even at body temperature; and in the hand the bird appears strangely soft and spongy. The precise composition of the fat varies to some extent with the diet of the bird, and with the part of the body where it is stored; but in those species studied (mostly passerines) it consists largely of unsaturated fatty acids, especially oleic, linoleic and palmitic acids, mostly stored in the form of triglycerides (McWilliams et al. 2004, Pierse & McWilliams 2005).

Although mainly passerines, this consistency across species is surprising given the diverse food habits of the species involved, suggesting that birds may be selective in their diets and in the fatty acids they lay down as fuel. Trials on captive birds have shown preferences for unsaturated oversaturated acids, and for short-chain over long-chain ones (McWilliams et al. 2004). Nevertheless, the composition of the diet has some influence on the nature of the fat and other body stores accumulated, particularly the ratio of fat to protein, and hence on the flight range (Prop & Black 1998, Jenni-Eiermann & Jenni 2003).

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