Conflicts at nestboxes
Birds aren't the only things that like to enter nestboxes. Predators and insects may also take up residence.
Birds such as sparrows and starlings often take over nesting holes used by tits. Most tits are able to defend a box successfully, provided that the intruder cannot get inside. A hole size of 25mm will exclude larger species. Do not fix a perch on the front of any box, as this will encourage intruders. Birds don't need a perch in order to use the box.
By putting up nestboxes with different sized holes, you'll cater for a variety of species. Please remember that sparrows and starlings are in serious decline and may need help even more than the tits. Do not place sparrow boxes too close to ones intended for other birds, especially house martin colonies.
Nestbox predators include cats, squirrels, rats, mice, stoats, weasels, woodpeckers and, in case of open fronted boxes, members of the crow family. As predators mainly hunt early in the morning, most people are unaware of their presence.
A metal plate fixed around the entrance hole may deter woodpeckers and squirrels, while barbed wired, gorse or rose clippings above and below the box will give some protection against most mammals, such as cats.
Insects and nestboxes
Bees, wasps or earwigs will, on occasions, take over nestboxes and there is little one can do to prevent it – apart from using insect sprays.
As many of the insects are useful food for birds, it is best to leave them alone. Insects often move in after birds have finished nesting. Any young found dead are likely to have died of other natural causes.
It is not unusual for the same type of insect to return to the box in subsequent years. Leave that box in situ and put up another one a few feet away. It is rare for both to be lost to insect invasion.