Avian species, including nocturnal birds, are highly dependent upon sight for survival. The avian eye displays the basic pattern of a vertebrate's eyes, but it incorporates many adaptations that have improved visual abilities. The avian eve is considered to be the finest ocular organ in the animal kingdom. The extremely large eye leaves little room in the head for anything else. Photoperiodicity has long been studied in birds as a means of inducing reproduction.
Day length is detected by the pineal gland.
The adrenal gland is located in the brain between the cerebella hemispheres and the cerebellum. It has numerous functions, one of which is photoreceptor and regulator of circadian rhythms. Some photoreceptors are located deep within the brain and are not dependent upon the eyes for photoreception. There is also evidence that melatonin influences reproductive hormonal cycles by affecting the pituitary gland. This has been very important in inducing poultry to lay eggs and to maximize egg production.