I'm going to come in on the dissenting side here and say, I prefer to hatch them in the cartons (large end UP). I believe I get better hatch rates this way than on their sides. I have not run a double blind clinical trial to support that belief, and it's possible that my better hatch rates are due to greater experience compared to my first few hatches (which were done on their sides). But here is the supporting evidence for my belief:
- When hatching on their sides in an incubator, the eggs do get kicked around. A lot. Especially if there are only a few eggs in the bator--versus full capacity. Lots of folks say this doesn't matter, but I have personally witnessed eggs start to hatch and then stop after having been kicked into apparently untenable positions. I have noticed that chicks tend to try to poke a hole on the "up" side of the egg... if the "up" side suddenly becomes "down" after the chick has already poked its hole, this seems to cause problems. I'm guessing here, but it seems likely that liquid from the egg might leak through the membrane and either drown the chick (if it is internally but not externally pipped) or cause excessive loss of fluid before it's ready (in the case of externally pipped eggs). For whatever reason, eggs that have been kicked into a different orientation after internal or external pip SEEM to me, based purely on undocumented observation, to have lower hatch rates.
- Yes, eggs hatch on their sides under a mother hen. The mother hen also holds them more-or-less in place with her body, checking on them frequently, and preventing games of egg soccer. In an incubator with eggs under capacity, there is no one to hold them in place or interrupt these games.
- Hatching in cartons forces the babies to hatch UP, which does in fact cause the hatch to take longer. This might be a "con," but I have not known it to ever prevent a chick from hatching once it got started. What it does do is force the babies to strengthen their legs while working to hatch, and the babies come out stronger and more coordinated than chicks that hatch on their sides. Again, this is a benefit inside an incubator where there is no mother to tuck them neatly and safely away and prevent injury. Uncoordinated chicks have a tendency to do things like get trapped under eggs in corners, or otherwise injure themselves.
On the downside, hatching in cartons does cause one potential issue: When chicks are left in the bator for more than 12-24 hours at a time, they will tend to start climbing on the eggs and can sleep on top of them... this can cause a pipped or zipping chick to get stuck, trying to push against the weight of another chick perched atop. Easy remedy is simply to remove any dry chicks twice a day, before they reach this stage.
By the way, humidity does not need to reach the under side of the eggs during the hatch. It only needs to reach the portions of the egg that are opened, to prevent them drying out from exposure to the air. I used to cut holes in the bottoms of my cartons, but I've never known it to make a difference, so I don't bother now. In fact, due to a mistake, I once hatched eggs in cardboard cartons that ended up soaked in water... so the eggs were sitting in wet cardboard throughout the hatch... and they ALL hatched.
I get 95-100% hatches (based on numbers that reach lockdown, not counting those who die earlier--I only WISH my rates were that high total! lol) using this method.
The reality is, eggs are pretty resilient. As long as the basics are right, you'll probably do fine. But if you're thinking about hatching in cartons, give it a shot. You might like it. Just be really, really sure you place the egg with the large end up, or you will drown your babies.
Good luck either way!