Photoperiodism

Day length (photoperiod) is the primary proximate cue for seasonal timing in birds because it is the only environmental signal that reliably predicts the approach of breeding or migration seasons across all years. Photoreception in birds occurs through the eyes but also through photosensitive neurons in the brain itself, acting directly on the hypothalamic-pituitary-gonadal axis. The circadian clock translates photoperiodic information into internal annual programs (circannual rhythms) that persist even in constant light conditions.