I agree with @Singing Gardener ... also allow for a clear flight path for the birds to access the nest box and for bluetits it should be at 5 -6 ft from the ground.
“I am not lost, for I know where I am. But however, where I am may be lost.” Winnie the Pooh
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wild edgesThe north west of south east WalesPosts: 5,519
Remove the peg on the front as it allows predator birds easier access but isn't needed for small birds. I'd sand the entrance hole too as it's a bit rough. A handful of wood shavings in the base makes it easier to clean in the winter and keeps the base dry.
And please don't put them where cats could get a paw in. Cats are excellent climbers and will take the baby birds if they can get access. Think fences and trees.
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You don't need to put anything inside. The birds will do that! You should clean them out each autumn though.