Complex bird clocks

The circadian pacemaking system of birds comprises three major components: (i) the pineal gland, which rhythmically synthesizes and secretes melatonin ; (ii) a hypothalamic region, possibly equivalent to the mammalian suprachi asmatic nuclei, and (iii) the retinae of the eyes. These components jointly interact, stabilize and amplify, each other to produce a highly self-susta ined circadian Output. Their relative contribution to overt rhythmicity app ears to differ between species and the system may change its properties eve n within an individual depending, for example, on its state in the annual c ycle or its photic environment. Changes in pacemaker properties are partly mediated by changes in certain features of the pineal melatonin rhythm. It is proposed that this variability is functionally important, for instance, for enabling high-Arctic birds to retain synchronized circadian rhythms dur ing the low-amplitude zeitgeber conditions in midsummer or for allowing bir ds to adjust quickly their circadian system to changing environmental condi tions during migratory seasons. The pineal melatonin rhythm, apart from bei ng involved in generating the avian pacemaking oscillation, is also capable of retaining day length information after isolation from the animal. Hence , it appears to participate in photoperiodic aftereffects. Our results sugg est that complex circadian clocks have evolved to help birds cope with comp lex environments.