The Spotted Egg

This morning in the yard behind my workplace, I spotted this:

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An egg.
Not just an egg, either.
It was also moving. With little breeze to blow so much as my hair, let alone the weight of an egg, I thought “maybe it has a bird inside!”
There are lots of birds around my workplace, so you never know. Not sure how it got there, in the middle of a Tarmac yard, without breaking, but never mind that. It’s a baby bird, trying to hatch!

I got some cardboard to scoop it up with (and a box that might make an ok temporary nest for it), and picked it up as carefully as I could. It rolled quickly a I was lifting it, and I nearly dropped it. It was also surprisingly light.
It was made of polystyrene.

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But why?
What is the point of a perfect egg of polystyrene? The only things I can come up with is that it’s used to model eggcups, or confuse snakes. What other use could it have?