108 University of California Publications in Geology [Vol.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 extinction in such forms as the raptors and 

 the scratchers. 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 40 

 suggests that it was during the Glacial Epoch that the migratory 

 instinct 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 extinct 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 instinct 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 

 Haliaetus has withdrawn toward the north into British Col- 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 and the morphnine eagles have withdrawn to the southward.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 



*° Allen, J. A., The geography and distribution of birds, Auk, vol. 

 10. No. 2, Apr. 1893. 



