24 University of California Publications in Zoology. (Vou. 
All this invasion of the higher altitudes oceurs when spring 
and summer are just dawning there, but when the foothills and 
plains below are becoming dry and barren under the July heat, 
no longer productive of the food-supply which they were in a 
condition to offer earlier in the season. I believe these relative 
conditions prevail throughout southern California. Without the 
mountains to accommodate the excess of bird population, which 
could not be supported in late summer on the withered lowlands, 
we would have far fewer birds in the spring. The ‘‘resident’’ 
species return to the lowlands when the cold begins to reduce the 
food supply in the mountains; and, what is also noteworthy, so 
oc 
do the ‘‘summer visitants,’’ which thus become transients for a 
few days in the fall as they pass back through the lowlands on 
their way south, or rather, southeastward. These latter, there- 
fore, undertake three distinct migratory journeys during the 
year: from their winter habitat northwestward to their spring 
breeding-place, from the latter up, and often northwards, to their 
summer feeding-grounds, and from there back down and then 
southeastward to their winter habitat. e 
And this brings up for consideration another phase of the 
subject of bird population and its modifying influences. Con- 
sidering only the breeding birds of the region, it would be inter- 
esting to know how many individuals there are in early June, 
before the population begins to augment any. It is, of course, 
impossible to get at an exact census enumeration. But I made 
estimates in different parts of the region, which will furnish an 
average from which an approximate idea can be obtained. The 
region under consideration is about 16 by 24 miles in extent, 
which would make its area 384 square miles. In some parts of 
this area, as around mountain meadows and along streams, I 
judged that there were twenty birds of all kinds to the acre; while 
over larger tracts on the mountain sides and desert slopes there 
may have been but one bird or even less to the acre. As a very 
conservative average let us say that there was one pair of breed- 
ing birds to the acre all over the 384 square miles. That would 
be 491,520 birds in the region at the time of least bird population 
(that is, just previous to the appearance of young-of-the-year). 
The following are some Transition and Boreal species, with 
