12 
WOODPECKERS IN RELATION TO TREES. 
uning the pole as a cantilever, loaded a! one end. it is found that it may be 
hollowed to the extent shown in the figure without decreasing its strength. The 
length of ordinal.' 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 it 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, if 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 lew 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. 
// results of tests of damaged poles; American Telephone it- Tdegraph Co., Zanes- 
ilc, Ohio. 
12 3 
Radkis of Pole, 
4 5 
Inches. 
Diagram showing the extent to which a pole 
can be hollowed by birds. without appre- 
ciably decreasing its strength. 
Pole No. 6390. 
Pole No. 6325. 
Pole No. 6372. 
ll 
Cedar 
44 
Cedar 
Cedar. 
44 
44. 
h 
iginal) 
Circumference below ground (minimum) 
Circoi Inl uf load 
35 feet 
35 feel 
35 feet. 
43) indies 
■W, inches 
42 \ inches 
42jiuhes 
401 inches. 
38 inches. 
31 inches 
t> inches 
31 A inches 
27 inches. 
8 inches 
(See note.) 
9 feet 6 inches 
point above ground 



< • round 

Ground. 
1.100 pounds. 
Earth. 
Earth 
1 foot 3 inch' 
1,300 pounds 
Rock 

7 inches 
5 inches 
ion i maximum ) 
ight. 
3.000 pounds 
2.000 pounds. 
