Yeah, a hen will lay a clutch of eggs, one each day for two weeks or so, then they all hatch about the same time. So it’s when they start incubation.
I wanted to mention that some people get messed up on counting. An egg does not have a day’s worth of development 2 seconds or 2 hours after you put it in the incubator. It takes 24 hours for it to have a day’s worth of development. And easy way to check your counting is that the day of the week you start them is the day of the week they should hatch. If you start them on a Friday, they should hatch on a Friday.
Another thing I wanted to mention is to not get too hung up on the 21 day thing. There are a lot of reasons eggs may hatch early or late; heredity, humidity, how and how long they are stored before you start them, and just plain differences in the eggs. A big difference is average incubation temperature. If your temperature is a bit high, they can be early. If it’s low they can be late. This temperature thing gets a lot of first time incubators.
I’ve had eggs hatch two full days early in an incubator and under a broody, though most of the broodies hatch pretty much on time. I adjusted the incubator temperature and got that a lot closer but how do you tinker with a broody? Some of that may have been something other than temperature. People on this forum have had eggs hatch a few days late in an incubator and under a broody. The 21 days is a target, not something exact. If mine hatch within 24 hours either way of the target time, I consider them right on time.
Hatching is not an instantaneous process either. There is a lot going on inside that egg the last few days. The chick gets itself into position and internal pips. That’s where it pecks into the air cell and learns to breathe air instead of living in a liquid world. It absorbs the yolk so it can live for maybe three days after hatch without food and water while its hatch mates hatch. It dries up the external blood vessels it no longer needs. It does something with that gunk it’s been living in so it dries fluffy instead of all gunked down. It’s a busy time. Who knows what else they are doing besides occasionally resting.
Some chicks do a lot of this between internal pip and external pip or even before. Some do a lot between external pip and zip. A few even wait until after zip and hatch to finish up.
Most of my hatches are over in less than 24 hours form first hatch to last, often in 18 hours. My last incubator hatch, I had one chick out a full 24 hours before any other egg had pipped, then they all hatched overnight. I’ve had some drag on for more than two full days, just one every now and then. It’s an exciting time but it can also be stressful.
Good luck with it. It’s fun.
