Why do you think incubation of the first egg starts when a hen goes to lay her second egg? Having watched hens laying eggs the posture to lay and the posture to hatch are very different and I suggest that the eggs already laid do not reach start incubation conditions when the hen lays subsequent eggs.When a hen goes to lay the second egg in the clutch, she is unavoidably starting to incubate the first egg laid. I have read nothing to date (and I've read a lot) to indicate that there is some physical or biological mechanism in the egg that can distinguish between this short incubation period and incubation proper. After dropping the second egg and sitting on it (and egg 1) till the bloom's dried, she stops sitting/incubating, and goes about her daily business, till she comes back the next day to lay the third one, and thereby temporarily incubates egg 1 again, and now egg 2 as well. And so it repeats until she deems the clutch complete, by which time egg 1 might have been partially incubated say 6, or 9, or 12 times. I think it quite possible that the earliest laid eggs in the clutch start to develop and then die early through this stop-start process of growing the clutch. It would be relatively trivial to test this hypothesis.
Clearly individual differences in behaviour will apply here. My hens vary in behaviour between about 20 mins to lay an egg and sitting there for around 2 hours. What triggers the start of incubation conditions to which you refer? Is it absolute, or might some hens trigger it prematurely sometimes or even normally? And what exactly are they?
This egg hatching business is complicated.
Thanks for the links; I shall read them in the morning when I am fresh. Need to sleep now; goodnight.
The first paper is full of detail but the only trigger I see there relevant (to the question: what initiates incubation?) is temperature, in particular, this bit: "Although the interaction among several physical agents during incubation influences in-ovo development, temperature has the strongest influence (Freeman & Vince, 1974; Decuypere & Michels, 1992; Meijerhof, 2009), because it can hinder, promote, or maintain embryonic and fetal development, as well as determine its rate and duration. Eggs are submitted to different temperatures from reception of the eggs at the hatchery until hatching. During storage, temperature is reduced to delay embryonic development. Subsequently, eggs are heated to reactivate embryonic development immediately before setting. "
good point. Cause of death is unknown at this point. Let's try to keep the focus on the first 5 days, because that's where the conflation with infertility resides.
The relevant bit of the second (which is a really useful document, thanks for linking!) is this (under Landmarks of Embryonic Development):
