_Recapitulation._

1. All birds of passage can exist in the climates, where they are produced.

2. They are subject in their migrations to the same accidents and
difficulties, that mankind are subject to in navigation.

3. The same species of birds migrate from some countries, and are resident
in others.

From all these circumstances it appears that the migrations of birds are
not produced by a necessary instinct, but are accidental improvements, like
the arts among mankind, taught by their cotemporaries, or delivered by
tradition from one generation of them to another.

XIII. In that season of the year which supplies the nourishment proper for
the expected brood, the birds enter into a contract of marriage, and with
joint labour construct a bed for the reception of their offspring. Their
choice of the proper season, their contracts of marriage, and the
regularity with which they construct their nests, have in all ages excited
the admiration of naturalists; and have always been attributed to the power
of instinct, which, like the occult qualities of the antient philosophers,
prevented all further enquiry. We shall consider them in their order.

_Their Choice of the Season._

Our domestic birds, that are plentifully supplied throughout the year with
their adapted food, and are covered with houses from the inclemency of the
weather, lay their eggs at any season: which evinces that the spring of the
year is not pointed out to them by a necessary instinct.