I know, it's hard to believe, but stick with me. When I'm collecting eggs I always leave an egg or two in the nest to remind the chickens to lay their eggs there again, because chickens, while they can't count, do know the different between zero eggs and some eggs.
But there's a second number that a chicken cares about -- it's between 10 and a dozen. If I don't collect the eggs, the chickens will look at them, and when they get a bunch of eggs, about a dozen, they'll start to sit on them to try to hatch them.
It's been pretty consistent with the heritage breeds, like the barred rock, a little less so with the white leghorns (the chicken in the picture at the start of this post) and I just realized this today.
Chickens can count. Here's chicken math: 0, (some eggs) 12