Circadian rhythmA cycle of activity which lasts (literally) 'about a day'. As most birds are affected by the alternation of day and night, their lives have circadian rhythms, but for coastal birds a tidally induced rhythm may be superimposed. There is a close connection between circadian rhythms and photoperiodism.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver