When do chicks usually internally pip?

[Nate Caskey]

When do chicks usually internally pip?

 

[chuckachucka]

I think the 'optimum' time would be some time during day 20, and then they would be externally pipped and zipped into day 21. However this varies massively depending on incubation temps, even with a broody hen sometimes. Generally I think it's around 24 hours before they are all the way hatched, something like that.