Introduction xix 



drive birds doAvn below or up above their normal vertical 

 habitat, so that it appears to me to be rash to base 

 any generalisations except on the position of birds during 

 the breeding season, when they are more or less necessarily 

 confined to the neighbourhood of the nesting site. 



In the following analysis the birds, 225 in number, 

 have been divided into those breeding at three different 

 levels : — 



(1) The plains, up to, but not including, the foothills. 

 This corresponds more or less to the Upper Sonoran 

 zone. 



(2) The foothills and mountain parks from about 6,000 

 to 8,000 feet, corresponding to the Transition zone of 

 Pifions and Cedars. 



(3) The mountains from about 8,000 feet to timber-Hne 

 at 11,500 feet, corresponding to the Hudsonian and 

 Canadian zones. There are only three birds which breed 

 regularly above timber-line. These are Leucosticte aus- 

 tralis, Anihus rvbescens and Lagopus leucurus. 



(a) Birds breeding on the plains and up to about 6,000 

 feet — 35 in number, or about 15 per cent. : — 



Podilymbus podiceps, Sterna forsteri, Hydrochelidon 

 n. surinamensis, Dafila aciifa, Marila americana, M^ 

 vallisneria, Ixobrychus exilis, PMlohela minor, Bartramia 

 longicauda, Colinus virginianus {Lophortyx calif or nianus), 

 Tympanuchus americanus, Buteo b. krideri ?, Aluco 

 pratincola ?, Dryobates bairdi, Cyanocitta cristata, Corvus 

 cryptoleucus, Icterus spurius, I. galbula, Quiscalus q. 

 ceneus, Coturniculus s. bimaculatus, Spizella pallida,. 

 Amphispiza b. deserticola, Peucoea cassini, Zamelodia 

 ludoviciana, Guiraca a. lazula, Spiza americana, Riparia 

 riparia ?, Stelgidopteryx serripennis, Vireo oUvacea, Vireo 

 belli, Icteria v. longicauda, Toxostoma bendirei, Thryo- 

 manes b. bairdi, Sialia sialis. 



