What causes a double shelled egg?

[nuthatched]

Yes, better known as the shell gland.
I think it's explained in this excellent video, which is worth watching regardless:

View attachment 3561064

Interesting, thanks for explaining. I would have though that only things that carry and give birth to live young would have organs we could call uteruses.

 

[South Dakota Guy]

Yes, better known as the shell gland.
I think it's explained in this excellent video, which is worth watching regardless:

View attachment 3561064

Great info! The video was very interesting.