A nest box, also spelled nestbox, is a man-made enclosure provided for animals to nest in. Nest boxes are most frequently utilized for birds, in which case they are also called birdhouses or a birdbox/bird box, but some mammalian species such as bats may also use them. Placing nestboxes or roosting boxes may also be used to help maintain populations of particular species in an area. The modern nest box was invented by the British conservationist Charles Waterton in the early 19th century to encourage more birdlife and wildfowl on the nature reserve he set up on his estate. Traditional nest boxes have existed in many civilisations, since antiquity. Nest boxes are getting more attention because increasing industrialization, urban growth, modern construction methods, deforestation and other human activities since the mid-20th century have caused severe declines in birds' natural habitats, introducing hurdles to breeding. Nest boxes can help prevent bird extinction, as it was shown in the case of Scarlet Macaws in the Peruvian Amazon. Construction General construction Nest boxes are usually wooden, although the purple martin will nest in metal. Some boxes are made from a mixture of wood and concrete, called woodcrete. Ceramic and plastic nestboxes are not suitable. Nest boxes should be made from untreated wood with an overhanging, sloped roof, a recessed floor, drainage and ventilation holes, a way to access the interior for monitoring and cleaning, and have no outside perches which could assist predators. Boxes may either have an entrance hole or be open-fronted. Some nest boxes can be highly decorated and complex, sometimes mimicking human houses or other structures. They may also contain nest box cameras so that use of, and activity within, the box can be monitored. Bird nest box construction The diameter of the opening in a nest-box has a very strong influence on the species of birds that will use the box. Many small birds select boxes with a hole only just large enough for an adult bird to pass through. This may be an adaptation to prevent other birds from raiding it. In European countries, an opening of 2.5 cm in diameter will attract Poecile palustris, Poecile montanus; an opening of 2.8 cm in diameter will attract Ficedula hypoleuca, and an opening of 3 cm in diameter will attract Parus major, Passer montanus, an opening of 3.2 cm in diameter will attract Passer domesticus. The size of the nest