Chapter 42 - Circadian rhythms

Abstract

Birds have evolved in a rhythmic environment in which the daily rotation of the Earth imposes daily changes in illumination, temperature, magnetic field, barometric pressure and other abiotic factors. This fact induces daily changes in biotic aspects of the environment as well, including the presence of predators, prey, parasites, food and mates. Thus, birds, as do most free-living organisms, have evolved an endogenous biological timing system, or biological clock, to both predict environmental changes and temporally coordinate multiple physiological processes. This circadian system comprises internal oscillators that are capable of generating rhythms of molecular, physiological and behavioral processes with a period of approximately (circa) 24 hrs (dian) in birds maintained under constant environmental conditions such as constant darkness of constant dim light, which are entrained to the 24-hr period via specialized photoreceptive element in the brain. The circadian system itself is comprised of at least 3 sets of circadian pacemakers residing in the lateral eyes, the hypothalamic suprachiasmatic nuclei and the pineal gland, which together integrate overt circadian rhythms. The role of the pineal gland in many birds, particularly in passerine birds, is an important feature of circadian organization through its daily and circadian biosynthesis of melatonin. Melatonin’s sites and mechanisms of action are discussed as well as the molecular mechanisms by which circadian rhythms are generated.