38 ANIMAL LIFE IN THE YOSEMITE 



To say all this a bit more simply, not alone food is necessary to the 

 bird life or the mammal life in our forests, but also safe places for rearing 

 young, and places of refuge when needed, for the grown-up individuals 

 themselves. Referring again to the relationships borne between certain 

 insects, birds, and trees: The White-headed Woodpecker (see p. 320) is a 

 species which does practically all of its foraging on trees which are living, 

 gleaning from them a variety of bark-inhabiting insects. But the White- 

 headed Woodpecker lacks an effective equipment for digging into hard 

 wood. It must have dead and decaying tree trunks in which to excavate 

 its nesting holes. If, by any means, the standing dead trees in the forests 

 were all removed at one time, the White-headed Woodpecker could not 

 continue to exist past the present generation, because no broods could be 

 reared according to the inherent habits and structural limitations of the 

 species. Within a woodpecker generation, the forests would be deprived 

 of the beneficent presence of this bird. The same, we believe, is true of 

 certain nuthatches and of the chickadees — industrious gleaners of insect 

 life from living trees. They must have dead tree trunks in which to 

 establish nesting and roosting places, safe for and accessible to birds of 

 their limited powers of construction and defense. 



We would go so far, even, as to urge that down timber, fallen and decay- 

 ing logs, are essential factors in upholding the balance of animal life in 

 forests. Certain kinds of chipmunks, and rats and mice of various kinds, 

 find only in fallen logs homes adapted for their particular ways of living. 

 And these chipmunks and other rodents have to do with seed scattering, 

 with seed planting, and with humus building, again directly affecting the 

 interests of the chaparral, of the young trees, and even of the older trees 

 of the forest. 



It is true that there are some kinds of birds and mammals which at 

 times directly injure trees to an appreciable extent. The birds of the 

 genus of woodpeckers called sapsuekers (see p. 327) drain the vitality 

 of the trees they attack. An overabundance of these birds would bring 

 disaster to the forest at large. An overabundance, likewise, of tree squirrels 

 (see pp. 202, 208) would probably play havoc with certain trees, beyond 

 the powers of these trees to meet the crisis. 



Just as in the case of the leaf-eating insects and of the kinglets in 

 the arboreal foliage, these birds and mammals of the sapsucker and tree- 

 squirrel category are kept in check by other, predatory birds and mammals. 

 In the Sierran woods are Great Gray Owls and Spotted Owls, Cooper 

 Hawks, Martens, and Weasels, levying upon the vertebrate life about them, 

 and each equipped by size, degree of alertness, or time of foraging, to make 

 use of some certain sort of prey. The longer we study the problem the 

 clearer it becomes that in the natural forests, which, happily, are being 



