Paths are provided both for pedestrians and for horses. Nevertheless, the destruction of ground plants by millions of trampling steps is tremendously effective in destroying the food and protective cover of such vertebrates as deer, chipmunks, and ground and bush-nesting birds. Yosemite has set the example in adoption of measures to minimize the sheer physical destruction to wild life resulting from concentrated human populations. The pigmy owl, bird demon of Yosemite Valley, nests in vacated woodpecker holes. Its continued presence depends on the availability of its particular housing needs.
People accidentally reveal concealment places of nests and of young to lurking enemies.
In Yosemite countless bird nests are discovered by humans who do not harm the eggs or young themselves but all unwittingly betray them to marauding blue-fronted jays and squirrels. This tends to increase the losses from these sources abnormally. Moreover, the jays are, if anything, more abundant than normal because man has brought them an increased food supply. In this particular instance the effect of development is to help the enemy and injure its prey. People discover the fawns of mule deer secreted in the meadows and so unconsciously help to reveal their hiding places to lurking enemies.