WOODPECEEES 323 



heart wood of a tree and thus hasten its ultimate disintegration. Water, 

 also, is thus afforded an easier entrance and this hastens decay. Event- 

 ually each and every tree must yield its place in the forest to seedlings. 

 The woodpeckers hasten this process of replacement, once the tree is dead. 



Many of the woods-inhabiting animals depend upon this woodpecker 

 to furnish them convenient nest holes or retreats. We have found Moun- 

 tain Chickadees and Slender-billed Nuthatches incubating their own eggs 

 in holes drilled in earlier years by the White-headed W^oodpecker; a 

 Sierra Flying Squirrel was found occupying an old AVhite-head's hole. 

 Probably, tree-dwelling chipmunks and perhaps California Pigmy Owls 

 also occupy holes of this woodpecker. 



The nesting cavities of the White-headed Woodpecker are gourd-shaped, 

 the entrance tunnel turning downward into a shaft which expands toward 

 the bottom. The internal dimensions vary somewhat, but the size of the 

 entrance hole is surprisingh' constant. Also, the symmetry in the outline 

 of the entrance is remarkable when it is recalled that an excavation just 

 begun is often higher than wide and has an irregular margin. One hole, 

 the first one listed below, measured 43 millimeters in four different direc- 

 tions — practically a perfect circle. The accompanying table shows the 

 measurements (in millimeters) of four nests of this species which were 

 studied. 



Height of top of opening from the ground.... 2760 2047 1480 3040* 



Vertical diameter of nest entrance , 43 47 47 37 



Horizontal diameter of nest entrance 43 42 42 37 



Top of hole to surface on which eggs or young 



rested 300 322 275 400 



Diameter of shaft 112 70-90 100 



Thickness of wall in front of shaft 40 105 15-35 65 



A'umber of eggs or young 5 4 5 5 



* " 10 ft. " 



Excavating is done by the removal of small chips or splinters, rarely 

 over 25 millimeters in length by 3 or 4 millimeters in diameter. Because 

 of the way in which chips are broken loose, the inside of the nest hole is 

 usually irregular in surface finish. The small detached splinters of wood 

 are, in the case of the White-headed Woodpecker, merely brought out to 

 the entrance and dropped to the ground directly beneath. No attempt is 

 made to keep the nest site concealed. An accumulation of such splinters 

 below a hole is an infallible guide to a newly dug nest. In the case of the 

 nest mentioned later as having been placed in a branch of a fallen black 

 oak, the chips were strewn for a distance of 3 meters on the ground along- 

 side of the prostrate trunk. 



