Egg without a shell

It may appear normal, but this duck egg is missing its shell and is covered only by a thin membrane.

Chickens lay eggs at any old time of day, but ducks are more organized: they almost always lay in the early morning. Sometimes if we let them out of the pen before normal, they're forced to lay eggs in the grass or other odd places. Like the porch.

Every now and then a chicken will lay an egg without a shell. It's usually when they first start laying that this happens -- as if all their parts aren't yet working correctly. But sometimes it occurs long after you'd think all their reproductive processes would be sorted out already.

I tried googling to find the cause, but mainly came up with, "It just happens sometimes." I hadn't noticed the ducks doing it too, until this egg.

As this page on Odd Eggs notes, when you pick it up, it feels like a water balloon.

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Half the fun of googling is finding the stuff that you didn't know you didn't know. For example I'd heard of century eggs (aka thousand-year-old eggs), and although they don't sound appetizing to me, I'd probably try one. Same with tea eggs. But Balut? Umm, no thanks.