Using gonadectomized quail it was possible to show unequivocally that the underlying photoperiodic response in birds (but not mammals) is driven by brain circuits that are switched on an off by day length. It demonstrated that measuring day length involved a daily (circadian) rhythm in photosensitivity with the birds being responsive to light particularly 12 and 18 hours after dawn.
Using gonadectomized quail, scientists were able to prove that the photoperiodic response in birds is controlled by brain circuits that are activated or deactivated by day length. This research showed that birds respond to light most strongly 12 and 18 hours after sunrise.