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Ducks do that while sleeping. If you observe ducks sleeping near a pond you'll typically see them sleeping with one eye open and one shut. That way they can still stay alert enough to watch for predators.

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I've had several pet birds and I've observed the same thing from them. I just assumed they were just pretending to sleep.

Your comment made me wonder how common it is with birds and it seems to be trait shared with all of them[1]. Pretty interesting. I asked my friend if her bird does that and it only seems to when it's not in a deep sleep yet, but eventually it will close both eyes.

[1] https://en.wikipedia.org/wiki/Unihemispheric_slow-wave_sleep

Gives a new meaning to the term, "sleep with one eye open at night." :)

Let me venture a guess: that expression comes exactly from observing animals keeping one eye open.

Haven't bothered to search for this, but do you know offhand if they also have a L-R hemisphere/body crossover?

All vertebrates have that, IIRC.

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