1908] Grinnell—Biota of the San Bernardino Mountains. 27 
The vast mortality in even the more slowly reproducing birds 
is a result of the equally large birth rate which is essential to 
provide the host of individuals factoring in this process. The 
phenomena of migration and limited habitats are results, as well 
as the evolution of new species through geographie variation 
because of isolation. 
To discuss an instance: The Audubon warbler is a common 
breeding bird of the Transition and Boreal zones of the San 
Bernardino mountains and other ranges of southern California. 
It winters in the Upper and Lower Sonoran zones of the nearby 
lowlands. Now, as far as I could see, during the summer months 
in the San Bernardino mountains, twice or thrice the Audubon 
warbler population could have gained a thrifty livmg. The 
birds became at least doubly numerous from the last of July on, 
when the families of young came into prominence. When and 
where does the pinch come which reduces this great augmenta- 
tion back to normal spring limits? 
I have on many occasions from year to year, at Pasadena in 
December and January, noted a great mortality among Audubon 
warblers, here, of course, in their winter habitat. This mor- 
tality occurs in dry cold weather, the dead birds being found in 
the morning on sidewalks beneath trees, or along hedges where 
presumably the birds had gone to roost for the night. The gen- 
eral impression among people is that these birds have been chilled 
to death in the night from exposure to the frost. But nothing 
has impressed itself more forcibly on my mind than the conelu- 
sion that well-fed birds do not die from exposure to cold. I 
have examined a great many of the dead Audubon warblers, and 
invariably found them emaciated, with not a trace of fat. This 
means that they had suecumbed from ill-nourishment because of 
the scarcity of the food to which they are adapted. The cold 
may have been the final factor ending the strugele of their im- 
poverished systems for sustenance. It is a common thing in the 
late autumn to see these warblers pecking away at withered and 
even hard-dried apples still clinging to the trees, or feeding on 
persimmons or belated figs. I haye even found weed seeds in 
the stomachs of Audubon warblers. All of this shows that as 
their normal insect «supply wanes past sufficiency, the same 
