158 University of California Publications in Zoology [Vol.12 



or abnormal closeness of the mandibles incurred in drying, and because 

 of the conical shape of the beak. The method of taking the measure- 

 ment here presented is to secure with calipers the diameter of the bill 

 from the highest point on the culmen along a line through the nostril. 

 This does not, therefore, take into account the extreme base of the lower 

 mandible, where it is difficult to find any constant point from which 

 to measure. 



It will be noted from the tables that, in both sexes, the average is 

 practically identical with the mid-point of range ; that the wing length 

 is least variable ; that males are more variable than females ; that the 

 bill of the female is relatively shorter and thicker than that of the 

 male, and its shape is much more constant ; that in the male the varia- 

 tion in the shape of the bill involves length much more than depth, 

 so that a wide range in degree of slenderness results, rather than in 

 actual volume; that in matter of general size of body (using the wing 

 and tail length as an index) females are nine per cent smaller than 

 males. 



Previously (1909, p. 281) the writer has given a table of measure- 

 ments of the then available specimens of Molothrus ater obscurus. 

 There were eleven males, from Arizona and southeastern California. 

 Comparison with the dimensions of thirty-one other males as here 

 presented shows remarkably close accord in all particulars save one, 

 due to depth of bill being taken to extreme base of lower mandible 

 instead of through nostril. It is of interest to note that among the 

 entire series of fifty -three cowbirds from the lower Colorado River 

 there is not one showing any closer approach to the Nevada form, 

 Molothrus ater artemisiae, than in the smaller series above referred to 

 and used originally for comparison. The apparent absence of M. a. 

 artemisiae from the Colorado Valley as a migrant is contrary to pre- 

 vious ideas as to its route of migration to and from the Great Basin 

 region. But what other route may be followed is still a matter of 

 conjecture. 



As regards color the Colorado series is remarkably uniform, except 

 in one respect: the steely reflections of the dark body plumage in the 

 male vary from deep violet to bronzy. This variation is exhibited 

 similarly in M. a. artemisiae and in M. a. ater. In obscurus, however, 

 the brownness of the head averages perceptibly paler than in the 

 Atlantic race, ater. It is in the female that obscut-us shows marked 

 color characters as compared with ater; and this has apparently been 

 overlooked in current descriptive works (as, for instance, by Ridg- 



