216 TERN.

They congregate in large flocks, but particularly in the
breeding season, when they are more than usually
restless; wheeling and redoubling their varied flight high
in the air, and uttering their loud screams in clamorous
confusion. Some of the species are described as breeding
on the shores, and depositing their two eggs upon the bare
rock; others lay three or four eggs in the dry sand;
and some nestle among the reeds and rashes in the
marshy borders of lakes. The young keep the nest a
considerable time after they have been hatched, not
offering to leave it until their wings have attained
sufficient length and strength to enable them to fly
with ease. ‘Though almost wholly web-footed, they are
seldom seen to swim. The moult occurs twice a year.
in all the known species.

One kind or another of these birds is met with in
almost every part of the world. Latham enumerates
twenty-three species, besides varieties.”