428 GRINNELL, The California Thrasher. ees 
San Quintin, not more than one hundred and sixty miles below the 
Mexican line in Lower California. 
An explanation of this restricted distribution is probably to be 
found in the close adjustment of the bird in various physiological 
and psychological respects to a narrow range of environmental 
conditions. The nature of these critical conditions is to be learned 
through an examination of the bird’s habitat. It is desirable to 
make such examination at as many points in the general range of 
the species as possible with the object of determining the elements 
common to all these points, and of these the ones not in evidence 
beyond the limits of the bird’s range. The following statements 
in this regard are summarized from the writer’s personal experience 
combined with all the pertinent information afforded in literature. 
The distribution of the California Thrasher as regards life-zone 
is unmistakable. Both as observed locally and over its entire range 
the species shows close adherence to the Upper Sonoran division 
of the Austral zone. Especially upwards, is it always sharply 
defined. For example, in approaching the sea-coast north of San 
Francisco Bay, in Sonoma County, where the vegetation is pre- 
vailingly Transition, thrashers are found only in the Sonoran 
“islands,” namely southerly-facing hill slopes, where the maxi- 
mum insolation manifests its effects in a distinctive chaparral 
containing such lower zone plants as Adenostoma. Again, around 
Monterey, to find thrashers one must seek the warm hill-slopes 
back from the coastal belt of conifers. Everywhere I have been, 
the thrashers seem to be very particular not to venture even a few 
rods into Transition, whether the latter consist of conifers or of 
high-zone species of manzanita and deer brush, though the latter 
growth resembles closely in density and general appearance the 
Upper Sonoran chaparral adjacent. 
While sharply delimited, as an invariable rule, at the upper edge 
of Upper Sonoran, the California Thrasher is not so closely restricted 
at the lower edge of this zone. Locally, individuals occur, and 
numbers may do so where associational factors favor, down well 
into Lower Sonoran. Instances of this are particularly numerous 
in the San Diegan district; for example, in the Lower Sonoran 
“washes” at the mouths of the canyons along the south base of the 
San Gabriel Mountains, as near San Fernando, Pasadena, and 
