A nest box or birdhouse is an artificial cavity made for birds or bats to nest inside. There are many birds that nest in the holes and crevices of old trees, but due to the felling and cultivation of trees for wood or paper, there are not many trees old and thick enough to house the cavities that would form in nature, where forest birds could nest. To make it easier for these species to find a suitable place, these artificial hollows are built, and thus a birdlife similar to that of primary forests can be achieved in secondary forests.

The size of the hole serves to avoid selection by size of the species. If the hole is larger it is easy for other larger birds to chase out the smaller species.

A tight hole also makes it difficult for predators to attack. Furthermore, the hole is not flush with the wall or tree, which prevents a snake from accessing the hole since it would have to move far away from the vertical surface and would not be able to grab it.

There are also nests for bats. These also nest in tree crevices as well as in caves.

The shape is different. They are open at the bottom and have no hole. Birds cannot nest there. These are usually placed more on walls than on trees.

These nests are not only useful for protecting nature, they can also be useful for biological control if they attract insectivorous species. Especially because the chicks grow very quickly and are very voracious.