Chick pipped on DAY 18! Help?

[Fleabuskitty]

Ok, I have an egg that pipped either late day 18 or early day 19 (it was about midnight when I noticed). I know bantams are notoriously early hatchers (although all 3 times I've hatched bantams and LF together they hatched at the same time), and this is an F2 bantam Phoenix x bantam Frizzle. I just took the eggs out of the turner Saturday night (end of day 17). Wednesday is day 21. I heard peeping when I opened the incubator to quickly turn the eggs I set 3 days behind the others (that's the annoying thing about turners - either all of them are in it or all of them out) but didn't think anything of it, seeing as all of the eggs in there are day 16 and up, until I happened to see a pip!
I am using a Little Giant still air incubator and doing everything by the book. This is the first time I've used this incubator, before I had a homemade incubator that just didn't hold humidity well and dry hatching never really worked for me. The thermometer that came with it read 99.5 - 100 the entire time (except, of course, when the incubator was opened) and the humidity has been about 50%, although yesterday I raised it to about 70%.
Anyway, does hatching early do any damage? Can the chick hatch on day 19 and still be healthy? Will there be an unabsorbed yolk sac? Or is early hatching just caused by faster development? I'm hoping it waits through tomorrow to hatch on day 20 at least, but...?
I know this probably has nothing to do with it, but chicks do communicate with each other when to hatch so... could it have mistaken the sounds of the two budgie chicks that moved into a brooder right next to the incubator on Saturday for chickens (they sound nothing alike, but maybe this is a stupid chick

?)
Really, it sounds mean, but if the chick doesn't make it or has to be culled it's not a huge loss since there are 16 other eggs of that breed, but I'm really just curious as to what exactly triggers chicks to hatch two days early. I've had plenty of chicks hatch a day or two late, even a six-day-late quail who is alive today, but never early.
Thanks in advance for any help!

 

[nurse_turtle]

After a chick pips internally, it has access to the air in the air cell. After using the oxygen available there, the chick's blood oxygen level begins to drop which signals their tiny chick brain to send signals to the muscles causing the chick's body to contract strongly. This muscle contraction causes the "pip". Just like any other species, you get 'em early and you get 'em late.

 

[Fleabuskitty]

Hmm, I didn't know about the oxygen thing. I knew they pipped internally and then externally shortly after, but I thought they knew to start trying to "escape" once they absorbed the yolk. Do you know what triggers a chick to pip internally?

 

[nurse_turtle]

I'm not sure exactly. I'm sure it's in their DNA somewhere. It should occur when the entire yolk is absorbed and maybe the little booger simply runs out of room in the membrane.

 

[Fleabuskitty]

Well, she hatched today and seems to be normal (if my feather sexing is correct, she is a hen). Her belly was bleeding a little earlier, but I've had lots of chicks hatch and bleed a little and she's fine now. And of course there are three more eggs of the same breed pipped

Maybe since they developed so fast they'll start laying earlier

 

[Fleabuskitty]

Well, one of her sisters hatched yesterday (day 19) evening, their brother hatched last night while I was asleep, then another female chick and two male chicks of the same breed hatched sometime between 9 and 3 today, along with a LF Buff Brahma mix (day 20), and three Buff Brahma mix eggs also pipped (there may be more that I don't see). I can't wait to see what hatches tomorrow when they are actually supposed to hatch!