The composition of depot fat in Western Sandpipers (Calidris mauri) was dominated by C16 and C18 fatty acids (palmitate, 16:0; palmitoleate, 16:1; stearate, 18:0; oleate, 18:1), which together comprised 80 to 90% of all fatty acids analyzed. Significant seasonal variation occurred in the relative proportion of specific fatty acids and the ratio of saturated to unsaturated fatty acids in relation to the migratory cycle of the sandpipers. In adults of both sexes, the proportion of 16:0 and 18:0 fatty acids decreased from winter (December) to premigration (March) to spring migration (May), whereas the proportion of 16:1 and 18:1 fatty acids increased over these same periods. Consequently, total unsaturated fatty acids increased by 34% between the winter and premigratory phase, and by a further 22% between premigration and spring migration. Therefore, biochemical modification of adipose tissue in Western Sandpipers begins during the premigratory period in preparation for long-distance flight but continues as migration progresses, perhaps reflecting a training component of physiological modulation for migration. Juveniles showed similar changes to adults in the composition of their depot fat during the “premigratory” period, even though most juveniles do not undergo hyperphagia or rapid fattening at this wintering site. Potential prey species collected from a wintering site in Panama had a higher proportion of saturated fatty acids, less 16:1 and 18:1, and more 18:0 fatty acid compared with those from a migratory stopover site in British Columbia, paralleling the seasonal changes in depot fatty-acid composition observed in Western Sandpipers. However, the fact that Western Sandpipers show an increase in level of unsaturation and in the amounts of 16:1 and 18:1 fatty acids in their fat stores on the wintering ground prior to migration suggests that seasonal variation in fatty-acid composition is not entirely diet dependent.
