There are many examples of photoperiodic mechanisms throughout the plant and
animal kingdoms. Seasonality, of course, is not the only aspect of the control exercised
by light in the animal's environment. Short-term rhythms are important in themselves,
e.g. in the control of egg laying rate in birds and the quality of light (its intensity and
colour). The "absolute" length of day (or night) at any time, its length relative to the
preceding or succeeding days, plus the changes in intensity of light marking the borders
of day or night, are all factors which seem able to act as a time signal to some internal
physiological mechanisms of the bird or mammal and thereby to assist its efficient
functioning. Were human beings able to imitate these signals successfully they would be
able to ensure the efficient physiological activity of ostriches at convenient times.