Introduction xix” 
drive birds down 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-line 
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, Anthus rubescens 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 acuta, Marila americana, M. 
vallisneria, Ixobrychus exilis, Philohela minor, Bartramia 
longicauda, Colinus virginianus (Lophortyx californianus), 
Tympanuchus americanus, Buteo b. krideri?, Aluco 
pratincola ?, Dryobates bairdi, Cyanocitia cristata, Corvus 
cryptoleucus, Icterus spurius, I. galbula, Quiscalus q. 
eneus, Coturniculus s. bimaculatus, Spizella pallida, 
Amphispiza 6. deserticola, Peucea cassini, Zamelodia 
ludoviciana, Guiraca a. lazula, Spiza americana, Riparia 
riparia ?, Stelgidopteryx serripennis, Vireo olivacea, Vireo 
belli, Icteria v. longicauda, Toxostoma bendirei, Thryo- 
manes 6. bairdi, Sialia sialis. 
