Fig. 3 shows the same nesting-box complete, provided with entrance hole and cover.
Fig. 4 shows an open nest-box for Flycatchers and a black Redstart.
The most important is that shown in 2 and 3 as it is specially arranged to suit Titmice.
Nest-boxes, and especially their holes, should, of course, be of different sizes, according to the birds that are to inhabit them. The opening is always round, and is of varying size according to the species. Many directions as to these are given in a paper by Baron von Berlepsch, “On the Protection of Birds Generally,” published by the German Association for the Protection of the Bird World, and also by publications of the Royal Society for the Protection of Birds in Hanover Square, London.