142 BIRDS OF LA PLATA . 
seek their food by preference on the bare level ground, 
or where the vegetation is most scanty. This species 
varies somewhat in habits, and seldom runs on the 
ground, and chiefly inhabits the desert plains, where 
the large grasses flourish. On one occasion when I 
was with an expedition on the pampas for several 
weeks, every day a number of these birds would 
gather and follow us; perched here and there on 
the tall grasses with their bosoms towards us, they 
often looked at a distance like large white flowers. 
Old gauchos have told me that fifty years ago they 
were abundant all over the pampas, but have disap- 
peared wherever the giant grasses have been eaten 
down and have given place to a different vegetation. 
Their note is a long, low whistle, the usual lan- 
guage of the Tenioptera; but in this species it is 
very like a human whistle, on account of which the 
bird is named Boyero (ox-driver) on the pampas. 
One severe winter great numbers of them appeared 
in the neighbourhood of Buenos Ayres, and it was 
amusing to see the dogs thrown into a great state of 
excitement by the low whistling notes heard per- 
petually from all sides. Every few moments they 
would start up and stare about them to ascertain 
where the deceptive call came from, and in spite of 
many disappointments they would occasionally all 
rush away, loudly barking, into the plantation, 
convinced that some person there was whistling to 
call them. 
The Black-Crown makes a somewhat shallow nest 
in a bush or large clump of grass, and lays four 
