I try to do what broodies or wild birds do, they all turn their eggs. If the yolk sinks into the small end of the egg or lays against the side of the egg, should the embryo be developing in cramped quarters, it can hatch with a cross beak, curled toes, spinal issues, can't absorb the yolk right before hatch, no feet, and a host of nutrient deficiencies. People that don't turn their eggs are asking for trouble in my humble opinion.
I am pretty obsessed about turning eggs.I start turning eggs on day 1 if I have collected them from my flock. Shipped eggs I wait until day 4. Turning is so important to keep the yolk centered so the embryo has room to move and grow, turning also refreshes the food source they have consumed around them. Not turning enough leads to deformities and malnutrition of hatchlings.
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Interesting! I'd love to read that study if you can find it, but you've convinced me that candling on the important dates (7D, 14D, 18/17/16D) doesn't hurt the embryo! I guess I'll turn on the turner on day 4 or so, to be on the safe side of either case
ok now I DEFINITELY don't want Guineas
You had me at PubMedMine go straight into the turner when I set them, unless they're shipped eggs and then I put them in cartons similarly to how I do lock down in cartons and I'll sort of gently rotate them in the carton to turn them a few times a day after a day or two of letting them sit. I've found with shipping-damaged air cells that rotating them to turn them does significantly better than tilting them as my automatic turners do, so those are always hand rotated.
As far as turning duration, here's a solid study on stopping turning early.
The important part being:
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They concluded in that article that there was no difference between turning until day 18 and stopping earlier at day 16, but it sounds like if you stop turning as early as day 13-14 there isn't a significant impact on viability of embryos.
And regarding turning during the first few days of incubation, there's this study where they turned eggs in four groups: only days 4-7, only days 8-11, like normal (days 1-18), and not at all. They found no significant difference in only turning days 4-7 and turning days 1-18. This one was much smaller scale than the above so take from that what you will, but it does sort of indicate that it isn't as important to turn them in the first few days of incubation.
So in conclusion, it seems to be most vital to turn day 4-13, but outside of that it does not seem to matter a whole lot. Breeds with unique needs like Seramas may not follow that rule, of course!
I dug those up, too. It looks like there was a slight increase in embryo growth rate and hatch percentage in their lighted incubator in the experiment, so maybe increase in vigor wasn't quite the right way to put it?: https://pubmed.ncbi.nlm.nih.gov/15222419/
That, of course, was 24/7 lighting from day 5 to 18, not just candling for a few minutes, so odds are pretty low you'd see any kind of change just for peeking every now and then, and especially not for candling on the normal day 7-14-18 (or 16 or 17) schedule.
As far as Guineas go... Let's just say there was more to it than just the eggsplosions that made it so when my last one died I never got any more of them.
I opened the door to find the roosters near that corner just staring. I moved the bedding away to find 3 Australorp chicks. Imagine my total shock. 
It’s amazing that they weren’t killed and all were healthy, growing into beautiful hens and rooster. Anyhow, it was summer…. higher humidity and temperatures. I learned on that day to ALWAYS triple check every nook and crannie when I have a girl in broody mode. Long story, I know… but it ties in to the egg-turning discussion. 
Holy cow! That's incredible!About 6 years ago, I moved my hens from one building to another. I left my roosters in the original building. Unbeknownst to me, my broody Black Australorp had a nest hidden in a far corner covered in bedding. She had been in the new building for over a week. Well, one day I went to tend to the roosters. I heard peeping from inside.
About 6 years ago, I moved my hens from one building to another. I left my roosters in the original building. Unbeknownst to me, my broody Black Australorp had a nest hidden in a far corner covered in bedding. She had been in the new building for over a week. Well, one day I went to tend to the roosters. I heard peeping from inside.
