462 NATATORES. STERNA. 
sent the fisirostral tribe of the Jnsessores, have a form and 
organization admirably suited to their particular economy. 
Being destined to pass the greater part of their existence in 
the air, their wings are of great extent, the tail long, and in 
general deeply forked (a shape possessed by most birds 
gifted with extraordinary power of flight), the body small 
and compact, the legs short, and with small feet, not calcu- 
lated to advance their progress in swimming, but sufficient 
for their support when occasionally reposing on the water. 
Such of the species as frequent the ocean subsist upon the 
fry and the smaller kinds of fish, which they take from the 
wing, by precipitating themselves upon such as rise within 
a certain distance of the surface of the water. In this act 
they descend with astonishing rapidity and force, and are 
often buried beneath the waves for several seconds. Others, 
more frequently found upon rivers and fresh-water lakes, in 
addition to fish, prey upon libellule, and other aquatic in- 
sects. The distribution of the different species is very ex- 
tensive, as they are found in all quarters and climates of the 
world. They are subject to a double moult, but the vernal 
change is confined to the region of the head, and the plu- 
mage of both sexes is precisely similar. The young, until 
after the first or second general moult, differ from the old 
birds. During the season of reproduction they congregate 
in large bodies, nestling closely together, generally upon the 
shingle, or lower parts of the islets they frequent. Their 
eggs are three or four in number. By the near affinity of 
the Gull-bilied Tern (Sterna Anglica) to some of the lesser 
species of dark-headed gulls, their connection with the genus 
Larus is evidently traced. 
