102 



University of California Publications in Geology 



[Vol. 9 



great variability displayed by the fossil phases of many birds, there 

 would seem to be some measure of propriety in such contention. 



ASIO 



When the study of the Rancho La Brea birds was first undertaken, 

 one of the most serious obstacles arising was the absence of comparative 

 material representing the Recent birds of America. The genus Asio as 

 found in North America today embraces two species, A. wilsonianus and 

 A. flammeus, both of quite extended range. There are now at hand 

 complete skeletons including the more characteristic limb bones of both 

 species so that a discussion of these parts may be safely undertaken. 



Tarsom etatars us. — The osteological characters of the tarsometa- 

 tarsus appear identical in the two species, the only difference lying in 

 the proportions of the bone. The specimen of wilsonianus at hand is 

 a female, that of flammeus is of unknown sex. In the owls, as in the 

 diurnal raptors, the female is commonly much larger than the male, 

 with the bones reaching the maximum of length and stoutness. The 

 female wilsonianus has a tarsus much shorter than in the one specimen 

 of flammeus, yet in all other dimensions the former bone is the larger. 

 Assuming the unsexed flammeus to be a male bird, a female would 

 doubtless exceed it in length and thus accentuate the difference between 

 the species. Assuming the specimen to be a female, the difference is 

 still a very positive one. A. flammeus seems thus to be a form with a 

 long slender shank as compared with its nearest ally, the short-shanked 

 A. wilsonianus. 



This difference of the tarsometatarsus is at least not incongruous 

 with the habits of the two birds. Asio flammeus is essentially a ground- 

 dwelling species. Its nest is on the ground ; its hiding place by day 

 is the long grass of open country ; its prey is sought in such places, 

 and after catching its prey the bird alights upon the ground to devour 

 its meal. The other species, in marked contrast, is a bird of the woods. 

 Although its prey may be sought in the open, its hiding place by day 

 is in the dense thickets along watercourses. In such places it builds 

 its nest, the male commonly hiding in the near vicinity. It seems 

 quite possible that the more ambulatory habit of Asio flammeus is 

 here seen reflected in the structure of the lower limb. 



In the asphalt beds typical tarsometatarsi of both species of Asio 

 are found. They seem to have hunted over common ground and to 

 have fallen to a common bait. 



