Last year four egs were laid, but only two hatched. Despite this, the parents tried to keep the two new chicks warm whilst at the same time incubating the non-viable eggs for a considerable number of days. Eventually one got broken on the nest and - if I remember rightly - the shell was consumed by the female. The fourth got broken during the short struggle by our ringers (or banders, as they're called in the US) to safely capture the eyass (young chick) for ringing.
In answer to a number of recent questions left in "Comments", Nick Brown (DWT) recently wrote the following. His answers are well-worth repeating here:
"Loads of questions! Here's a few quick answers: By starting incubation right away, owls produce young of different ages. This is an adaptation to a fluctuating food supply (vole and mice numbers are well known to cycle from abundant one year to scarce the next). Often the smaller chicks die (or are eaten by their bigger siblings) and the brood size is reduced to match the food supply. Peregrines by comparison have a much more reliable food supply and usually rear all their 4 (even occasionally 5) young. So synchronised hatching makes sense.The reason the peregrines are covering the first and second eggs is to keep them warm (during what has been a cold snap of weather). Serious incubation - when she gets right down and opens up her hot 'brood patch' on the eggs -won't happen until at least the third and probably the fourth egg is laid.The last egg is often a bit smaller than the first three, so probably is slightly less likely to produce a strong youngster than the others. Some birds sit on addled or infertile eggs for weeks after the 'expected' hatching date. However, if say two of the four eggs hatch, the parents will start feeding these immediately while continuing to brood the other two eggs (and the small chicks). This happened last year on the cathedral. Gradually they just ignore the unhatched/dud eggs and focus their attentions on the two chicks."