MARSH-BIRD 115 
YELLOW-SHOULDERED MARSH-BIRD 
Agelzus thilius 
Black; lesser upper and under wing-coverts yellow; bill and feet 
black; length 5.5 inches. Female pale brown, striated with black ; 
eye-mark white ; paler beneath; smaller. 
Tus bird is abundant everywhere on the pampas, 
and does not migrate, but inhabits marshy situations 
in summer, building its nest amongst the rushes, 
and in winter ranges over the country. The male is 
entirely of an intense black, except the shoulders, 
which are pure yellow ; the female is dull grey with 
fuscous markings, and, as was long ago remarked 
by Azara, the grey-plumaged are very much more 
numerous than the black individuals. The young 
birds are like the females, and possibly do not acquire 
the full black plumage until the second year, which 
would account for the great number of grey birds. 
These birds are extremely sociable, being seen in 
flocks all the year round, even during the breeding- 
season ; in winter a great many males separate them- 
selves from the females, and are found associating 
together in flocks of from thirty to forty individuals. 
They feed on the ground, keeping to the moist 
borders of marshes during summer; they avoid 
woods, but occasionally alight on trees, where they 
all sing in concert. The song, when an individual is 
heard singing alone, is, though limited in its range, 
very sweet, some of the notes being remarkable for 
their purity and expression. The bird sits on a rush 
