Nictitating membrane: noun, zoology; ‘a whitish or translucent membrane that forms an inner eyelid in birds, reptiles, and some mammals. It can be drawn across the eye to protect it from dust and keep it moist’ (Oxford Dictionary).
When I was taking photographs of one of my local Crow friends last week, I caught a couple where its nictitating membrane was showing well, so thought I’d share.