296 Phillips, The American Black Ducks. [julv 



In the autumn of 1910 Mr. W. W. Brown, Jr., collected some 

 birds for me in the lake region near Mexico City. In the collection 

 was a large series of At^as diazi, heretofore very rare in museums. 

 There were six males and seventeen females, taken near Lerma in 

 the early Fall. 



Upon examining this series I was struck at once with the very 

 marked sexual difference shown in the skins, a difference which 

 does not appear to have been properly noticed. The species was 

 first described (Ridgway, Auk, 1886, p. 332) from an immature 

 female and then later by F. Ferrari-Perez (Pr. U. S. Nat. Mus., 

 1886, p. 127) from an adult male, as well as an immature female. 

 In the latter paper the female was said to be very similar to the 

 male except that the streaks of the lower parts were narrower. 

 The sexual color differences of bills and legs were not noted. 



It is at once apparent that this bird is widely different from the 

 other American Black Ducks. 



Sexual dimorphism. The most noteworthy sexual differences 

 are as follows. The male is somewhat larger than the female, 

 more especially in the wing measurement. The upper mandible 

 of the male is colored a dull olive or yellow-olive, while that of the 

 female is dusky along the center and brilliant orange on the sides. 

 Conversely the feet and legs of the male are bright orange and those 

 of the female a dull orange color. The under parts of the males 

 are barred with irregular blackish V-shaped markings, while the 

 females are all more or less finely streaked with a lighter dusky 

 shade. This difference of appearance is due to a different feather 

 pattern, particularly on the front and sides of the breast. (See 

 plate.) The typical female feather has a dark central wedge at the 

 tip, while the male feather is edged with brown and patterned 

 with a V-shaped sub-terminal black area. 



Individual variation. Both the anterior and posterior ends of 

 the speculum are framed by a black band inside, and a light band 

 outside. The posterior white band, formed by the tips of the 

 outer secondaries is fairly uniform, but the anterior white band is 

 very variable. In some cases it is narrower and mixed with buff 

 or dusky ; and in three cases it is absent altogether and replaced by 

 a very faint dusky line. These last are all females. We see thus 

 a tendency to vary in the same direction as will be pointed out 



