Okay bear with me.
I've kept chickens for years, and I've never experienced this phenomenon.
I've currently got three broodies, one has just hatched five lovely chicks.
The other two are sitting on turkey eggs in neighbouring nest boxes. The nest boxes are plastic fruit crates, so they have raised sides all round. One hen is on two eggs, the other on three. They come off the nests for a bit every day, and often swap nests, as they're side by side.
Here's what's weird - the number of eggs in each nest keeps changing. Somehow, one or more of the turkey eggs (bigger than normal hens' eggs) is being moved from one nest to another. I know broodies will instinctively roll any eggs they see into their nests, but how the hell are they lifting them over the sides of the crates?
Has anyone experienced this?