Where in the avian brain is the photoperiodic response triggered to release GnRH for stimulating brown egg production?

Unlike mammals, where light perception is limited to the retinas of the eyes, birds possess a highly advanced system of non-retinal photoreception. The primary trigger for the photoperiodic response is located in the hypothalamus, a region in the basal part of the avian brain. Within the hypothalamus, specialized neurons express deep encephalic photoreceptors (containing rhodopsin-like pigments). Because the skull of a chicken is thin and porous, light of specific wavelengths—particularly orange-red spectrum light (630-640 nm)—can penetrate directly through the feathers, skin, and bone of the skull to reach these deep brain receptors. Upon stimulation by an increasing photoperiod (e.g., 16 hours of light), these receptors signal the hypothalamus to secrete Gonadotropin-Releasing Hormone (GnRH), which triggers the pituitary gland to release LH and FSH, stimulating ovarian follicle development. Blind chickens can still lay eggs in response to photoperiod changes due to this mechanism. Farmers can study photobiology in the Poultry Encyclopedia, purchase specialized LED poultry lighting on Poultry Plaza, monitor daily rates on Poultry Rates, and list high-production layer flocks on Murghi Mandi.