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