Photoperiodism is the word used to describe the way organisms react to light and darkness. These factors can change on a daily, seasonal or yearly basis. Photoperiodism triggers hormones to be released from the pituitary gland and these hormones control migration and reproduction. In other words, if you want to trick a migrating bird into flying the wrong way, you would reproduce spring light conditions in the autumn or vice versa.

Photoperiodism is an important factor in migration but other environmental stimuli also have a role, such as the availability of food and the temperature. Photoperiodism is used in the poultry industry to control the production of eggs and the size of the birds.