Circannual cycles and photoperiodism

Birds have evolved in an ever-changing world in which seasonal changes impose abiotic and biotic factors in a rhythmic fashion, predictably creating both beneficial and deleterious selective pressures. As a result, birds, like most free-living organisms, have adapted circannual biological clocks to maximize their fitness, coordinating complex annual phenologies of reproduction, molt, migration, and metabolism. At least some of the annual cyclicity in birds is due to the expression of endogenous circannual oscillators, which are revealed in laboratory conditions as rhythms with periods of approximately but not exactly one year. These circannual rhythms are entrained to 365 days by the presentation changing durations of the light cycle or photoperiod. Integrally related to entrainment of circannual cycles is the photoperiodic control of annual cycles. The mechanisms by which both circannual rhythms and photoperiodic time measurement are generated have been studied extensively. Specialized photoreceptors residing in multiple loci in the brain are responsible for entrainment and photoperiodic induction of reproduction, molt, migration, birdsong, and other processes, and structures associated with circadian organization have also been implicated. While the mechanisms of circannual rhythms are not known, molecular mechanisms underlying photoperiodic time measurement and annual cycles of reproduction involve circadian clocks in the hypothalamus and pineal gland, involving the expression of circadian clock genes, and the regulation of thyroid-mediated cellular processes. These mechanisms are compared and contrasted with those underlying annual cycles in mammals and fish.