When do eggs start rocking?

[Weber Girls]

This is my first time hatching chickens. What day do the eggs usually start to rock back and forth? Mine started on day 17. Do they usually start then, sooner, or later?

 

[LTygress]

It all depends on the chick, and there really is no set day.

The embryo is moving pretty much from the first day that it begins to develop. I've candled eggs that have a single dark spot about the size of the end of a pencil eraser, and seen the dark spot move - ever so slightly - without me moving the egg at all. So it really depends on when the embryo becomes big enough, and strong enough, to make a movement that will rock the entire egg.

I've had eggs that rocked only about halfway in, and I've had eggs that never rocked at all, until the chick was at the very end of the hatch and zipping out. Likewise, I've had chicks that screamed the moment they made an internal pip, and I've had one chick that I honestly never even knew had hatched in the incubator, because he never made a sound. No rocking, no chirping, nothing. The incubator stays in my room, so I was surprised to check it the next morning when I woke up and not only see a fully hatched chick, but he was dried AND walking, and more than ready for the brooder!