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".