Fleas and parasites reside in the nests of most birds, and remain to infest chicks that hatch the following year. When the breeding season is over, old nests can be removed and the box cleaned out as long as the nesting residents have stopped using the box.
Bird-protection law permits the removal of unhatched eggs between 1 September and 31 January in England and Wales, and 1 August and 31 January in Scotland. If dead eggs are found in the nestbox they must be destroyed promptly during this period only and they cannot be kept or sold. It is quite normal for a few eggs to fail to hatch, or for some young to die. For example, blue and great tits lay up to 14 eggs to allow for such losses.
Wear surgical gloves and a dust mask when cleaning out nestboxes. Use boiling water to kill any remaining parasites, avoid using insecticides and flea powders and let the box dry out thoroughly before replacing the lid.
A handful of clean hay or wood shavings placed in the box once it is dry after cleaning may entice mammals such as woodmice to hibernate there, or birds may use it as a roost site.
Main image: Wear gloves to clean a nestbox © Billy McPherson