Long-distance migration in birds is characterized physiologically by periods of rapid fattening and lipogenesis, and increased desaturation of fatty acids stored in adipose tissue. We investigated seasonal, age- and sex-related differences in activities of two lipogenic enzymes, fatty acid synthase and Delta(9)-desaturase, in relation to migration in the small, Arctic-nesting western sandpiper (Calidris mauri). Migration, and associated lipogenesis and fattening, involved marked upregulation of these enzymes in this species. However, this increase in enzyme activity was only seen in actively migrating birds during spring migration, when fatty acid synthase and Delta(9)-desaturase levels increased by 53% and 113%, respectively, compared to Iron-migrating birds. There was no change in fatty acid synthase enzyme activity during the premigration period, even though body mass of adult birds increased significantly during this period. Similarly, there was no increase in Delta(9)-desaturase activity during premigration, despite the fact that birds increase the proportion of monounsaturated fatty acids in their fat stores at this time. We suggest that upregulation of lipogenic enzymes is required to support high rates of mass gain (0.4 g day(-1)) during short (1-4 day) periods at stop-over sites. However, slower rates of mass gain (0.09 g day(-1)) over several weeks prior to migration can be achieved without any increase in tissue-specific enzyme activity.
