So intentional or unintentional? My two cents, first knock was unintentional, second was given the adults' subsequent behaviour since it happened on the 9th.
Does it really matter how it got to where it's position now is? What really matters is how it got to be in the condition it appears to be in (an egg of two colors, light and dark). Did a half shell from a previous hatch get pushed onto the light colored egg or visa versa(the three hatches were close together lots of shells around),as happened at James River Bridge site in 2008. Under these circumstances the chick can't peck through the two shells and sufficates.Also how can we tell what egg in the clutch it is, Number 1, 2, 3 or 4?
The why is interesting and important from my perspective - it tells me how Hurricane and Brooklyn are developing and evolving as parents. Last year because it was so wet, wasn't a good year to get a sense of how these birds individually and as a pair behave. Determining how they deal with eggs will let us know for future years what they are going to do or what it means when they have done something. Think of Pa in the Netherlands, poor bird just won't give up. If Hurricane and Brooklyn were the same way, we would intervene so they weren't wasting their reserves/resources on a lost cause. Knowing that within a week they will cut their losses means that in the future we can be pretty sure they are going to handle it appropriately.
Your scenario about the chick suffocating because there were two egg shells wouldn't appear to apply here because the depression in the eggshell, presumably the expansion of the "pip" is free and clear as seen on numerous photos and video clips. It is just as likely that the eggshell was thinner, perhaps because of the delay in laying since her first three eggs hatched out fine, so the depression was caused by one of the parents in which case the chick was probably in trouble long before hatching.
And there is no way to tell the eggs apart by lay date, just that in this case because of the very long lag between when egg 3 and egg 4 was laid, it would be highly unlikely that egg 4 would hatch out at the same time as its sibs - it was between 4 and 6 days younger, which is a very long time in incubation. The egg itself was pretty dark as well, the colour tends to wear off the longer it is from laying and from being "polished" by feathers and pebbles during incubation.
So what happens is important because it will let us better understand what is going on in future years. It could also help us in the future to identify if there is reproductively a problem with this pair - like Hurricane's small size or incubation/brooding behaviours, which is different for every pair. Hurricane can produce four chicks - she did with Zeus but he was exceedingly experienced. With Brooklyn she hasn't managed to do the same - last year the weather was horrendous so its out of the mix for now. This year more normal for both of these birds so if half of a sib's shell is on the fourth egg, was it just an accident/luck or is it that Hurricane and Brooklyn don't pull out eggshells once they hatch? Princess for example pulls/spins them out from under her almost before the chick is free, but she's bigger and maybe the shells bug her more when she's on the chicks/eggs. Hurricane and Brooklyn don't have egg shells around much, either eaten, carried away or stomped into little pieces. Will that change as they get more experienced? Or will that just be their "way". There's nothing wrong with any of this, it's all just what peregrines do naturally, so working out how the birds respond will help us when new situations arise.
