12 



WOODPECKEES IN RELATION TO TREES. 



Assuming the pole as a cantilever, loaded at one end, it is found that it may be 

 hollowed to the extent shown in the figure without decreasing its strength. The 

 length of ordinate between shaded area and right-hand edge of the diagram represents 

 the thickness of outer shell which must be sound. For example, at 10 feet from 

 the ground if only 2 inches of the outer shell are left, the pole will be approximately 

 as strong as though it were solid. This grants the bird permission to build a house 



about 6 inches in diameter. The higher up the 

 pole it goes the larger can be its nest without 

 injury. If, however, the attack is less than 4 

 feet from the ground the pole will be appre- 

 ciably weakened. 



This illustration neglects the damage done 

 by the entrance into the pole, or the subse- 

 quent decay which may follow, and assumes 

 that the bird builds its nest exactly in the 

 center — a condition not always found in fact. 

 On the other hand, it assumes that the pole 

 has a uniform moisture per cent through- 

 out its length and that the outer fibers at the 

 ground line are perfectly sound. These condi- 

 tions seldom, i.f ever, exist in practice, as the 

 pole, particularly if it has been set for a year 

 or more, always contains more moisture near 

 the ground than at any point above it, and the 

 sapwood at the ground is invariably more or 

 less decayed. Hence, the birds could peck 

 even larger holes than those shown in the sketch 

 without increasing their damage. 



The engineering department of the American 

 Telephone & Telegraph Co. made a few tests 

 in 1908 near Zanesville, Ohio, to determine the 

 effect of woodpecker attacks upon the strength 

 of poles. These tests were made by fastening 

 a rope around the top of the pole and pulling 

 with a block and tackle to which a dynamom- 

 eter was attached. In 9 cases out of 12 the poles broke at the ground line and not 

 at the points attacked by the birds. The examples in the accompanying table are 

 typical. 



Typical results of tests of damaged poles; American Telephone & Telegraph Co., Zanes- 

 ville, Ohio. 



12 3 4 5 6 

 Radius of Pole, Inches. 



Diagram showing the extent to which a pole 

 can be hollowed by birds without appre- 

 ciably decreasing its strength. 



Pole No. 6372. 



Material 



Wires 



Length 



Circumference, at ground (original) 



Circumference below ground (minimum) 



Circumference, point of load 



Diameter bird hole 



Distance of hole above ground 



Distance of load point above ground 



Broke off at 



Under corrected load 



Set in 



Top reflection (maximum) 



Butt reflection (maximum) 



Estimated breaking weight 



Cedar 



44 



35 feet 



43.', inches 



3.s.\ inches 



31 inches 



Clinches 



9 feet 6 inches . . 



25 feet 



Ground 



520 pounds 



Earth 



2 feet 11 inches. 

 1 foot 3 inches.. 

 1,200 pounds... 



Cedar. 



35 feet 



42 J- inches. 

 42 inches.. 

 31 '. inches. 



24 feet. 



26 feet. 



Rock 



5 feet 7 inches. 



5 inches 



3,000 pounds . . 



Cedar. 

 44. 



35 feet. 

 40J inches. 

 3S inches. 

 27 inches. 

 (See note.) 



Ground. 

 1,100 pounds. 



Earth. 



2,000 pounds. 



