Taken seconds apart a few days ago in Key West, these two photos illustrate how the fiery orange-red eye of a juvenile Yellow-Crowned Night Heron is suddenly and fleetingly obscured by its nictitating membrane. In fact, it happens so quickly, I'm never aware of it while shooting birds--only afterwards when I'm reviewing files on my monitor.
According to the Cornell Lab of Ornithology, "Birds have what is known as a nictitating membrane or 'third eyelid'. This is a clear eyelid, closest to the eyeball. It is transparent and can close and protect the eye when hunting."