Why do the periods of the arrival and departure of migrating birds vary in different years?

Because they depend entirely on the changes of the seasons. Thus, the meanest rustic, in regard to the summer birds of passage, is aware, that cold weather prevents the arrival of these messengers of spring; and that the early arrival of our winter birds of passage, indicates a proportionally early winter.

Why is the arrival of these summer birds to be partly prognosticated by the leafing or flowering of particular trees or plants?

Because the same circumstances of temperature which retard the birds, also check the progress of vegetation. As the state of vegetation depends on the temperature of the season, and the life of the insects, (the food of birds) on the state of vegetation, we may safely conclude, that the movements of the phy tivorous (vegetable-eating) and insectivorous birds must be dependent on the condition of plants. - Fleming

Why is torpidity also called hybernation ?

Because it is evidently designed to afford animals protection against the cold of winter.

Actual torpidity in birds is very rare; yet the few instances on record establish the fact, while they point to the numerous resources of Nature in extreme cases, to preserve existence.

Why were Seven of the migratory birds formerly called the Seven Sleepers?

Because it was then supposed that many birds,which, it is now known, unquestionably migrate, retired to some secure retreat, and there remained dormant during the winter. - Jennings.

Why does the early arrival of wild geese and ducks, and other migrating birds from the north, in the winter portend that a severe season is approaching?

Because the early appearance of these birds is most likely caused by severe frost having already set in, at their usual summer residence. - Jennings.

Why do the bird-catchers in the neighbourhood of London procure males only on the first arrival of this bird?

Because the males of many species of migrating birds appear to perform their migrations a few days before the females; and this is remarkably the case with the nightingale. The females do not make their appearance for a week or ten days after the males. - Fleming.