XII.  The knowledge and language of those birds, that frequently change their climate with the seasons, is still more extensive:  as they perform these migrations in large societies, and are less subject to the power of man, than the resident tribes of birds.  They are said to follow a leader during the day, who is occasionally changed, and to keep a continual cry during the night to keep themselves together.  It is probable that these emigrations were at first undertaken as accident directed, by the more adventurous of their species, and learned from one another like the discoveries of mankind in navigation.  The following circumstances strongly support this opinion.

1.  Nature has provided these animals, in the climates where they are produced, with another resource:  when the season becomes too cold for their constitutions, or the food they were supported with ceases to be supplied, I mean that of sleeping.  Dormice, snakes, and bats, have not the means of changing their country; the two former from the want of wings, and the latter from his being not able to bear the light of the day.  Hence these animals are obliged to make use of this resource, and sleep during the winter.  And those swallows that have been hatched too late in the year to acquire their full strength of pinion, or that have been maimed by accident or disease, have been frequently found in the hollows of rocks on the sea coasts, and even under water in this torpid state, from which they have been revived by the warmth of a fire.  This torpid state of swallows is testified by innumerable evidences both of antient and modern names.  Aristotle speaking of the swallows says, “They pass into warmer climates in winter, if such places are at no great distance; if they are, they bury themselves in the climates where they dwell,” (8.  Hist. c. 16.  See also Derham’s Phys.  Theol. v. ii. p. 177.)

Hence their emigrations cannot depend on a necessary instinct, as the emigrations themselves are not necessary.

2.  When the weather becomes cold, the swallows in the neighbourhood assemble in large flocks; that is, the unexperienced attend those that have before experienced the journey they are about to undertake:  they are then seen some time to hover on the coast, till there is calm whether, or a wind, that suits the direction of their flight.  Other birds of passage have been drowned by thousands in the sea, or have settled on ships quite exhausted with fatigue.  And others, either by mistaking their course, or by distress of weather, have arrived in countries where they were never seen before:  and thus are evidently subject to the same hazards that the human species undergo, in the execution of their artificial purposes.

3.  The same birds are emigrant from some countries and not so from others:  the swallows were seen at Goree in January by an ingenious philosopher of my acquaintance, and he was told that they continued there all the year; as the warmth of the climate was at all seasons sufficient for their own constitutions, and for the production of the flies that supply them with nourishment.  Herodotus says, that in Libya, about the springs of the Nile, the swallows continue all the year. (L. 2.)