BIRDS OF YOSEMITE NATIONAL PARK 81 



at night. The only hawks whose habits are to be questioned seriously are 

 Cooper's hawk, pigeon hawk, goshawk, prairie falcon, duck hawk, and 

 sharp-shinned hawk. Owls generally are beneficial, and the Pacific homed 

 owl is a destroyer of such pests as jackrabbits, cottontails, and pocket 

 gophers. 



Analyses of the stomachs of 121 hawks and owls gave evidence that 

 these birds ate little but ground squirrels, rabbits, and mice. 



Game birds which are hunted for food and sport may be killed only 

 during the open season established by law. 



MIGRATION OF BIRDS 



With seasonal changes, many birds become uneasy and, following some 

 instinct, they move southward or northward, mountainward or valleyward 

 as the case may be. They travel singly or in small or large groups. 



During migration, birds take certain general directions to their desti- 

 nations, guided, perhaps by instinct and topographical features. Little is 

 known regarding the forces which guide birds in their migratory flight. 



During migrations many birds die or are killed. Thousands of weak- 

 lings drop from exhaustion when long flights are taken without a stop. 

 Storms often carry great flocks out of their courses. High buildings take 

 their toll. Dead birds are found often at the base of the Washington 

 Monument. Thus, in a way, migration strengthens the race of birds. The 

 weak die. The strong survive. It is a form of natural selection. 



Ornithologists, or bird students, state that at least nineteen species of 

 shore birds breed near the Arctic Circle and visit South America in the 

 winter. More than a hundred species leave the United States to spend the 

 winter in Central or South America. 



ACKNOWLEDGMENTS 



In preparing statements relating to distribution and status, frequent 

 reference has been made to Grinnell and Miller, 1944, "The Distribution 

 of the Birds of California," Pacific Coast Avifauna, 27: 1-608. This excel- 

 lent and thorough work has greatly lightened our task. Persons interested 

 in additional information on the birds of Yosemite should refer to Grinnell 

 and Storer, 1924, Animal Life in the Yosemite, Berkeley, University of 

 California Press, xviii + 752 pp. An earlier publication of Yosemite Nature 

 Notes, "Birds of Yosemite" (January 1938) by Beatty and Harwell, in- 

 cluded a check list of 203 species with descriptions of 70 of the most com- 

 mon. For a more complete guide to birds of California and the West, 

 A Field Guide to Western Birds by Peterson (1941) and/or Birds of the 

 Pacific States by Hoffmann (1927) are recommended. 



We wish to acknowledge the generous help and encouragement of 

 Douglass Hubbard, Associate Park Naturalist, Yosemite National Park, 

 without whose stimulation this book would not have been prepared. Wayne 

 Bryant, Junior Park Naturalist, Norman Herkenham, Assistant Park Nat- 

 uralist, and Walter Fitzpatrick, also of Yosemite, have read the manu.script 

 and have made many helpful suggestions. David Johnston of the University 

 of California Museum of Vertebrate Zoology checked the accuracy of the 

 technical names. 



