BLACK-HEADED DUCK 95 



DISTRIBUTION 



This rare bird seems to be confined to the basin of the La Plata, and central Chile. It has been found 

 in Paraguay (H. von Ihering, 1904; Bertoni, 1913) and has been recorded from the following Provinces 

 of Argentina: Tucuman (Lillo, 1902); Rioja (Giacomelli, 1907); Mendoza (British Museum); and 

 San Juan (American Museum of Natural History). In Buenos Aires Province it is not common (P. L. 

 Sclater and Hudson, 1889; A. H. Holland, 1892; Hartert and Venturi, 1909; C. H. B. Grant, 1911; 

 E. Gibson, 1920; Wetmore, MS.). It has been recorded from Uruguay (Tremoleras, 1920) and from 

 Rio Grande do Sul, southeastern Brazil (H. and R. von Ihering, 1907). In Chile, Lane (1897) met 

 with it on the Rio Pilmaiguen. The British Museum has specimens from Santiago. Presumably the 

 species is found only in the central parts (Quijada, 1910). When Lord William Percy was in Santiago, 

 a monk in charge of some birds in a monastery there told him of seeing Heteronetta(?) in Cautin 

 Province in December (1923?). Percy obtained a twenty-year-old skin at Concepcion and tells me 

 that the bird must be very rare in that region. 



GENERAL HABITS 



Less is known of this species than of any other South American duck, excepting 

 perhaps the Brazilian Merganser. Systematically it certainly has very little in 

 common with the surface-feeding ducks with which it is placed. Its affinities are 

 uncertain, not only on account of the structure but also because of its habits. The 

 male has no bulla ossea; the trachea is simple, although the middle third of it is soft 

 and swollen and about double the diameter of the remainder, as it is in some diving 

 ducks. On the other hand it has the slender body, narrow, light bill, and small feet 

 of a surface-feeder. 



Suggestive of relationship to the diving ducks was the discovery by Dr. A. Wet- 

 more (in litt.) that the male has a large oesophageal throat-sac. This is not like the 

 tracheal air-sac found in the spiny-tailed ducks (Oxyurinse). It is situated at the 

 upper end of the oesophagus, is 50 or 60 mm. long, and in the region of the lower 

 jaws is projected into two sacs which connect with the mouth. The bird must have 

 some general resemblance to the spiny-tailed ducks, because according to P. L. 

 Sclater and Salvin (1876) hunters in Chile used to confound it with the Ferruginous 

 Ruddy Duck (Oxyura ferruginea). It has the curious full, loose neck-skin of the 

 ducks of that genus. It dives as actively as any true diving duck, though the hind 

 toe lacks the broad lobe characteristic of the subfamily Fuligulinse. The tail- 

 coverts are nearly as long as the rectrices, and the wing is rather short. Dr. Wet- 

 more thinks this species may be intermediate between the surface-feeders (Anas) 

 and the spiny-tailed ducks (Oxyurinse). 



These birds are fond of reed-beds, and are exceedingly hard to observe because 

 they do not fly about, but prefer to hide when disturbed. They are also very shy and 

 retiring in their habits (A. H. Holland, 1892; C. H. B. Grant, 1911; E. Gibson, 

 1920). 



Some interesting notes, made by Dr. A. Wetmore, have been recently sent to me; 

 I quote freely from them. The birds were found by him only on the canadones on the 



