244 NATATORES. 
sient visitors in the spring and autumn months, the 
cold regions of the fur countries being their usual 
place of resort during Summer. 
The Canvass-back is the famous Duck which is 
generally considered by epicures as the finest of all 
the Duck family,— its flesh being thought to possess 
a peculiarly agreeable flavor, which no other fowl can 
claim. ‘The most common winter resort of these cele- 
brated Ducks is the Chesapeake Bay and the rivers 
and streams belonging to it, such as the Susquehanna, 
Patapsco, Potomac, and James rivers. Here they 
sometimes assemble in flocks of such great numbers 
as to cover the surface of the water for acres in ex- 
tent, and when they rise suddenly the noise of their 
wings resembles thunder. The abundance of their 
favorite food (a species of Valisneria), a grass-like 
plant which grows to the height of a few feet above 
the water, the roots of which seem to form their main 
sustenance, is evidently the great attraction for these 
birds, as of later years their numbers appear to have 
decreased, while at the same time the plant has be- 
eome less abundant. These Ducks are often seen 
feeding in company with several other species, such 
as the Black-headed Duck, the Widgeon and the Red- 
headed Duck. They all appear to live upon the same 
plant; the Canvass-back and the Black-head diving 
to obtain the roots, while the Widgeon and the Red- 
head prefer the leaves. The Canvass-back has also 
been found on the waters of the Hudson, and upon 
some of the western rivers; but its chief winter 
