Janet Baraclough wrote:
Can't be calcium deficiency in one and not the others, since they eat the
same,
Are they all the same age? Calcium deficiency is age related; it
happens when a hen has laid so many eggs her reserves are used up

Yes, they are all betweeen 1 and 2 years old, and the bantams are the
youngest (we got them last spring as 6 week olds - they were /tiny/!)

One of the bantams does lay better than the other, I think (it's a bit hard
to tell, as the eggs are identical!), but I'm still strongly suspicious that
this odd egg may be an attempt by Chickeny (the original, now-non-laying
chicken) to try again. There was a suspicious brown feather underneath the
egg! (the bantams are white)