Vascularity of the nictitating membrane
A striking feature in some birds is the vascularity of their nictitating membranes. This is most easily seen in large birds with big eyes and transparent membranes (Figure and video)) but vessels are visible in most birds if they are looked for carefully. The rich blood supply of the nictitating membrane may have a role in maintaining respiration of the cornea when access to atmospheric oxygen is blocked during sleep.
Arcades of blood vessels in the nictitating membrane of black-necked storks (left female, right male).
Engorgement of blood vessels in the nictitating membrane of a female black-necked stork, played at 10% speed)