I've occasionally noticed something about the eyes on some of the birds I observe: milky or filtered texture.
According to the Cornell Lab of Ornithology http://www.allaboutbirds.org, birds have what is known as a nictitating membrane or “3rd eyelid”.
This membrane is a translucent or clear eyelid, closest to the eyeball. A bird will close these membranes over their eyes but can still see through them to protect their eyes when needed, such as in flight, while hunting, or to dive under water.
This Northern Flicker may have been trying to shield his eyes from flying bits of bark as he probed for insects.
