108 University of California Publications in Geology (Vou.7 
percentage of migratory forms such as the anserines and the 
pygopodes. These birds by their migratory habits are rendered 
largely immune to the effects of climatic change that might 
have brought about extinetion in such forms as the raptors and 
the seratchers. Ten of the fifteen extinct species recorded from 
Fossil Lake belong to genera which are at present non-migratory 
in the region. 
Whether or not these genera were migratory during Pleisto- 
cene time is, of course, a matter of pure conjecture. Allen*® 
suggests that it was during the Glacial Epoch that the migratory 
instinet was indelibly impressed upon birds by the pronounced 
seasonal contrast prevailing at that time. Whether the instinct 
was at that time incipient or real, it seems proper to conclude 
that those genera which now display it are the ones which would 
have profited by its initial operation and have escaped extinction. 
There presents itself, then, the very potent suggestion that 
the relatively small proportion of extinet forms represented in 
the Fossil Lake horizon is due to the fact that many of the 
genera there represented possessed or else developed the migra- 
tory mstinet and were preserved except as influenced by other 
factors. 
The remaining four horizons may more properly be com- 
pared as to age upon the basis of percentage of surviving species, 
and such comparison bears out the conclusions reached by Os- 
born in his study of the mammals. 
Causes of Extinction of Birds——After a consideration of the 
varied and in many respects remarkable avifauna of Pleistocene 
times, it is natural that the causes of extinction of these forms 
should hold an important place in our attention. Why should 
we now have but two eagles in southern California where five 
once flourished? Why does but one condor remain of the five 
species found fossil? The large phase of the variable Pleistocene 
Haliaétus has withdrawn toward the north into British Col- 
umbia and Alaska, while Phoenicopterus, the ciconids, Polyborus 
and the morphnine eagles have withdrawn to the southward. 
The gigantic Teratornis disappeared, leaving no near relative 
40 Allen, J. A., The geography and distribution of birds, Auk, vol. 
10, No. 2, Apr. 1893. 
