At one time shops in Sweden used to sell a triangular nest box with 18 cm sides, 16 cm internal depth, and an inner floor with 6 cm sides. On the one hand it was so small and cramped that no birds could manage to stay in it, and on the other hand the entrance hole was so high up that it was too narrow. There was an imminent risk of birds bceoming stuck in it. These nest boxes are still set up in many recreational areas. Some books recommend making one's own triangular boxes and sawing away the the top triangular portion of the front side instead of drilling an entrance hole. In such a case one has to be careful to saw off sufficient for the hole not to be too narrow. Considering some of the extraordinary nest sites chosen by birds each year there is no good reason why the boxes should not be whatever shape you chose so long as they are large enough, weatherproof, and have a suitable entrance hole and are placed in a secure position. 1 have field tested triangular nest boxes for around 15 years and provided that the round hole bored into the front is large enough for the birds'use the results have not differed from conventional boxes.
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