Bulletin - United States National Museum - si-pddr - Smithsonian ...

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 cafiadones
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, behaving
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, becoming
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 became
normal. There was no tracheal air sac.