In cattle they can select for ‘calving ease’ bulls, or rather bulls that throw calves with a shorter gestation, basically they are cutting days, maybe even weeks off of gestation and since the calf grows significantly each day, you get smaller calves.  Wouldn’t surprise me if birds had a similar gene plus or minus a hot corner of the incubator.  I have quail so they only get 24 hours in the incubator but I don’t like leaving them in there for longer than a couple hours.  Part of the reason is my incubator runs 80 percent humidity and the chicks never get dry.  The other is pathogen exposure.  Ever notice we culture bacteria in an incubator in the lab?  Yeah, a bird incubator has the exact same conditions: warm, humid and organic material (food).  So if you haven’t thoroughly sanitized your eggs and incubator before incubating, having vulnerable babies flopping around and going beak first into something nasty.  I’m a neophyte incubator, started last spring with shipped eggs, I faithfully kept the little buggers in there 12-24 hours for fear of shrink wrapping.  Also had 20 percent death loss in the brooder by day 3 in both batches.  They just never grew and then died.  Working diagnosis is a clostridium: spore forming bacteria ubiquitous in soil and feces, produces a toxin that kills before you see any symptoms save malaise and belly pain in some cases.  I was feeding a 30 percent protein starter but I switched to 21 percent (quail) mostly for local availability but also clostridium multiplies rapidly in the presence of excess nutrients or the absence of competitive bacteria (normal GI flora gets wiped out by antibiotics, ever heard of C. Difficile?).  The only other change I made was getting chicks out of the incubator within a couple hours.  And using home raised eggs.  And yes, I do clean the incubator between batches. I’ve hatched 10 or so batches since last spring and now have had 0 brooder chick mortality.  Chicks weakened by shipping, ration too ‘hot,’ too long in a contaminated incubator, a complete fluke?  Who knows, but at least I can move on to puzzling out something else!