I've read that it is not newly hatched chicks but chicks after they internal pip but before they external pip. And that the eggshells need to be touching to have that effect so it may be movement more than just them chirping. If it is movement then chicks crawling around after hatch could have an effect too but it would be delayed if they are not touching before hatch. The effect would only start after they first ones hatch if that makes sense and it would take time for them to get ready to hatch.
I've had several hatches under broody hens and in the incubator where the entire hatch was over within 16 hours of the first one hatching. That implies some hatching coordination. I've also had hatches under a broody hen and in the incubator take closer to 48 hours from the first one to the last to hatch and all eggs were started at the same time. Like many things with chickens I take this to be a case of can, maybe, might, could possibly rather than an absolutely, without doubt, 100%, everytime.
