Why do birds often lay eggs and mammals often give birth in spring?

Birds and mammals often time their reproduction to coincide with spring because it is an evolutionary adaptation that maximizes the survival of their offspring. By launching a new generation during this season, parents ensure that the period of greatest vulnerability—between birth or hatching and maturity—occurs when environmental conditions are most favorable, specifically when temperatures are warmer and food sources are plentiful.12

This seasonal timing is regulated by photoperiodism, the physiological response of an organism to changes in the duration of daylight. For many species, increasing day length in late winter and early spring acts as a primary environmental stimulus. This light exposure triggers the pituitary gland to release hormones that stimulate the growth and activity of the gonads, effectively preparing the animal for mating and reproduction.314

While light is the major cue for species living away from the Equator, other factors such as nutrition and internal biological rhythms also play critical roles in ensuring that reproductive cycles are synchronized with the most advantageous times of the year.342