o be associated with changes in the liver.
In a recent study by one of us (Bartell) on blackcaps (Sylvia atricapilla) exhibiting Zugunruhe, the liver circadian clock fundamentally changes. In particular, timing of molecular clock components is altered, and the rhythms of many clock proteins fluctuate between higher maxima and lower minima. The circadian clock in the liver becomes stronger and more dominant as greater emphasis is placed on metabolism for flight as opposed to other behaviors or physiological processes. This study was conducted using birds that spontaneously exhibited Zugunruhe under constant day length, indicating that the changes in the liver clock are brought about by internal circannual cues. Because birds exhibiting migratory behavior do not eat at night, the changes in the liver clock and in liver metabolic pathway regulation are not due to differences in feeding time or to differences in the nutrient content of their food.
In migrating birds, feeding remains a daytime activity, but energy-requiring flight switches to nighttime. Very few studies consider such time-of-day effects, and instead focus on global changes in metabolism associated with migration. The study on blackcaps exhibiting Zugunruhe mentioned above suggests that during the day, the clock in the liver primes the bird’s body for more efficient nocturnal flight by inducing increases in PPARγ (see Sidebar: Circadian Regulation of Fat Metabolism, above left). Conversely, during the night, the liver clock ramps up the energy-burning process to power flight by inducing increases in PPARα. Thus, the birds’ circadian and circannual clocks are integral components to their extreme migratory physiology and behavior, referred to as migratory syndrome.
These experiments underscore the complex relationship between the circadian clock, fat metabolism and migration. We don’t yet fully understand this system or how it changes during migration. Clearly, an internal circannual clock controls seasonal changes in both fatty acid metabolism and the liver clock, and these changes are associated with migratory state. Further work is needed to unravel the causal relationships.