‘The warblers were not relying on environmental cues:? the changes in their behaviour were prompted by an internal clock consisting of two fundamental rhythms.? Circadian rhythms, corresponding to the twenty-four-hour cycle of the Earth’s rotation on its axis, regulate daily changes in metabolic rate, body temperature and level of alertness.? Circannual rhythms, corresponding to the annual cycle of the Earth’s orbit round the sun, control changes in behaviour associated with reproduction, moult and migration.’
William Fiennes, ‘The Snow Geese’