All Your eggs in one basket
There is an old saying “Don’t put all your eggs in one basket.” Obviously, it means that if you drop your basket you lose the lot, whereas with the eggs separated out in more than one place there is a better chance some will survive in the event of an accident.
Some birds follow this principal too. Blackbirds and thrushes, for example, raise two or three broods of young between April and August. Each time they lay four or five eggs. By spreading out 12-15 eggs in different nests at different times, these birds ensure that some of their young have a good chance of making it through their most vulnerable time.
Blue tits take a different approach and put all their eggs – up to 16! (but usually 8-12) – in one box, one time only each year. It is risky, and timing is critical. It largely has to do with their food supply;tiny caterpillars, and coinciding the appearance of all those hungry mouths with the greatest abundance of little moth larvae in the tree-tops and bushes, some time in May.
WildwoodWatch has nest-box cameras in the park. One is showing a blue tit brooding her eggs in a box attached to the big oak beside the rat barn, where usually you can watch events unfold in real time.
Believe it or not, in a small space like this the eggs (and the young when they hatch) may be in two layers and the hen bird periodically swaps them around! If they were too spread out she would not be able to fit them under her small body and keep them warm.