Endocrine regulation of fueling by hyperphagia in migratory birds.

To support migratory endurance flight, birds accumulate large amounts of fat by hyperphagia (fueling). Whereas the factors influencing migrants' motivation to fuel are well described, the physiological mechanism regulating fueling is largely unknown. Hormones are likely involved and arguably the best studied with respect to food intake and fueling is corticosterone. Corticosterone has a permissive effect, as blocking the hormone's actions prohibits efficient fueling. There are no indications, though that corticosterone stimulates fueling, and some studies even observed negative correlations between corticosterone level and food intake and speed of fueling. The latter is unexpected as slow fueling could reduce the overall speed of migration. To test the causality of these negative correlations, I non-invasively increased circulating corticosterone levels in captive migrants and determined its effect on food intake and fuel accumulation. Neither food intake nor fuel accumulation differed between corticosterone-treated and control-treated individuals. This indicates that corticosterone does not hamper food intake and fueling during stopovers, nor does it stimulate these processes. Promising alternative candidates for the regulation of migratory hyperphagia are 'appetite regulating' hormones secreted by the adipose tissue, gut, or gastro-intestinal tract. The advance of next-generation sequencing will facilitate a bottom-up approach when investigating these.