32 



He made use of every opportunity of carefully examining woodpecker 

 holes, though his work was often attended with considerable difficulty, 

 because the tree in question had to be felled and cut up so that its 

 interior might be thoroughly examined. Without in the least antici- 

 pating the true state of affairs, he was actuated simply by the wish 

 that his investigations might lead to the discovery of principles which 

 he could follow in the construction of a nesting-box which should be 

 generally useful a box, in short, which would take the place of the 

 natural woodpecker's hole in every respect. 



To his surprise he discovered after examining several hundreds of 

 woodpecker holes, that they were all formed on a uniform plan. All 

 the holes of the black, green, and grey woodpeckers, as well as those of 

 the various spotted woodpeckers, in spite of difference in size, were 

 constructed on exactly the same principles. The problem of a satis- 

 factory nesting-box was brought very much nearer its solution by 

 this important discovery. It was now merely a question of producing 

 an imitation of these woodpecker holes, which should be true to nature 

 and serve the generality of birds, and the problem was satisfactorily 

 solved after many failures. 



(c) Manufacture of the Nesting-boxes. 



There were many difficulties connected with the imitation of the 

 peculiarly-shaped woodpecker holes, as they were to be faithfully 

 copied, down to the smallest details. The accompanying illustrations, 

 taken from photographs of the longitudinal sections of natural 

 woodpecker holes, show clearly the method of their construction. 



The opening is always circular, and is of unvarying size with each 

 species, i.e., with the lesser spotted woodpecker, 1J inch ; the greater 

 spotted woodpecker, If inch ; the green woodpecker, 2f inches ; the 

 black woodpecker, 3f inches.* 



A very important point has been observed, namely, that by a wise 

 provision of Nature the opening of all the holes is inclined upwards 

 to a certain angle (4 deg.) in the interior, so that the rain cannot 

 penetrate. 



* The measurements of the holes of the black woodpecker are correct when the 

 openings are round. When they are more or less oval, as is often the case, 

 especially in eastern Europe, the horizontal diameter is smaller, and the vertical 

 diameter is longer than is stated above. 



