Circannual Cycles and Photoperiodism

Annual cycles of exposure of the northern and southern hemispheres to solar irradiation due to the Earth's axis and the asymmetric revolution of the Earth around the sun results in dramatic changes in the environment in temperate and circumpolar regions, called "seasons." Seasons vary in the length of photoperiod, ambient temperature, and precipitation as well as changes in biotic environmental factors, such as food and competition. Birds exhibit profound annual cycles in their phenology due to their complex life cycles and heavy investment on rearing of young, resulting in annual cycles in courtship, reproduction, migration, molt, and other aspects of their biology that are strictly regulated by endogenous circannual clocks, synchronized by the length of the photoperiod and other factors. Tropical zone birds also exhibit annual cycles according to their seasons, but the mechanisms by which these are regulated are unknown. The mechanisms by which temperate zone birds detect changes in photoperiod include extraretinal photoreceptors residing in the preoptic area, lateral septum, and tuberal hypothalamus. The mechanisms by which birds measure photoperiodic time depend upon the physiological process being measured as well as the species of bird. Annual cycles of reproduction appear to be independent of pineal melatonin, in sharp contrast to the situation in mammals. Instead, photoreceptors within the tuberal hypothalamus itself and possibly preoptic area entrain a circadian clock within this area of the brain and induce transcriptional changes downstream to effect hypophysial outflow of gonadotropins and reproductive recrudescence. In contrast, annual changes in structures associated with bird song appear to be regulated by pineal melatonin, at least in part. Thus, annual cycles in birds are regulated by a complex integration of diverse photoreceptors and oscillators. This is an area ripe for future molecular and physiological analyses.