The whites of their eyes

On a quick walk near Granville Island, I stopped to admire this scene:

Mallard, Canada goose and green reflections.

Later, looking over my photos, I noticed something. Look at their eyes.

The mallard's eyes are open. The goose has a white screen over his.

Now the duck has followed suit.

The white screen is the nictating membrane.  This is a transparent or translucent membrane that protects the eye from injury, or from drying out, much as our eyelid does. Unlike the eyelid, it allows the possessor to see while it is in place. So these sleeping birds can still watch for danger (hawks, eagles, dogs)  while they sleep.

A better look at the eye covered by the membrane.

See it again, in this next goose:

With eye bared.

With the nictating membrane covering it.

Birds, reptiles, amphibians, and fish have these membranes. So do we, vestigially; they don't work. (It's a pity; we wouldn't need swim goggles or sunglasses with them on.) A few mammals have them; sea lions, manatees, polar bears; animals that live or hunt in extremely hazardous conditions, where the membranes are life-savers. Platypuses and aardvarks have them, too. In the aardvark, an ant eater, they keep the ants out. Cats and dogs have the remains of one, only semi-functional.

For a bird, the membrane is useful, not only to keep an eye out for danger, but to protect the eye from strong winds and dust, especially during a dive. And forget trying to sneak by an eagle (if you're a mouse) while his eyes look like they're closed! He'll see you anyhow; you're toast!

And they protect the eyes from the insistent beaks of hungry chicks, pecking at anything within range. (Another reason I wish I'd had one; how many toys have kids jammed into my eyes? Too many.)

I like this, from Wikipedia:

Woodpeckers tighten their nictitating membrane a millisecond prior to their beak impacting the trunk of a tree in order to prevent their eyes from leaving their sockets.

The nictating membrane crosses the eye horizontally, from the centre outwards. I'm sure I've seen this in ducks; now I will be looking out for it.