BIRDS OF ARGENTINA, PARAGUAY, URUGUAY, AND CHILE 83 

 HETERONETTA ATRICAPILLA (Merrem) 



Anas atricapilla Merkem, in Ersch u. Gruber, Allg. Encyc, sec. 1, vol. 35, 

 1841, p. 26. (Buenos Aires.) 



The curious black-headed duck was encountered only in the cafia- 

 dones on the Estancia Los Yngleses, south of Lavalle, Province of 

 Buenos Aires. The birds frequented pools surrounded by rushes 

 where the water was from 2 to 3 feet deep, and were shy and retir- 

 ing, so that it was difficult to observe them. On my first encounter 

 I found two pairs in a small pond in company with coots and other 

 ducks. To my astonishment the black-headed ducks dived when 

 startled and disappeared like so many grebes, evidently seeking the 

 shelter of the rushes. Another pair was observed on November 9, 

 1920, when I was fortunate in securing the male, though both birds 

 dived instantly at the flash of the gun. The female disappeared and 

 was not seen again. When in the water the birds suggest ruddy 

 ducks, though the tail is not held at an angle as in the genus 

 Erismatura. 



Black-headed ducks were evidently breeding during the first week 

 in November. Females noted swam about with heads erect, behav- 

 ing like other ducks. Males followed them or faced them with 

 neck drawn in and throat puffed out, at intervals raising the point 

 of the bill and giving a low note quah quah^ barely audible at 45 

 meters. It is possible that in diving quickly the birds use wings as 

 well as feet, but on this point I was not certain. The species is 

 widely distributed but is not common as it was not encountered 

 elsewhere. 



The colors of the soft parts in the male that I secured were as 

 follows: Top of bill behind nostrils, line of culmen, nail and space 

 behind it black; base of maxilla shading from Rocellin purple 

 anteriorly to vinaceous buff toward feathers; rest of maxilla deep 

 Dutch blue; mandible tilleul buff, becoming deep Dutch blue at 

 base; iris bone brown; front of tarsus and toes deep olive buff, be- 

 coming neutral gray on sides, joints, and webs. 



The specimen taken was an adult bird in breeding condition. At 

 the back of the mouth on either side was a vertical slit 12 mm. long, 

 forming the entrance of a thin-walled cheek pouch that extended 

 backward, and to a slight degree downward, for about 25 mm., 

 above and external to the hyoidean muscles. This sac was evidently 

 capable of considerable inflatiom For at least the anterior half it 

 was overlaid by a thin fascia of muscle, probably a portion of the 

 cucullaris. In addition, the upper end of the esophagus is full and 

 large, with thin walls that are pouched outward, evidently capable 

 of expansion. Midway of the neck the walls of the esophagus be- 

 came normal. There was no tracheal air sac. 



