At various times we've reviewed a few facts about the migration of birds in which the question was raised: why do they migrate? There are two principal reasons. One reason is that those summering in the north need to move away from soon-coming cold weather with short winter days providing very little sunshine.
While departure times from northern points usually follow close schedules, in some way humans cannot understand, migrating birds have been given ability by the Creator even to know about weather changes long before our weathermen and will advance their migration time to get away before heavy storms move in.
Then, after some six months in the sunny south lands, the birds get restless and seem to sense that the ice and snow on ponds and streams of the north are melting. Days are getting longer again, and an invigorating climate for hatching and raising their young compel them to make the return trip northward.
The second principal reason for their migrating is that food supplies are all-important, and somehow they instinctively know that plenty of fresh, nourishing food will be waiting for them at the other end of their journey. For instance, as though anticipating the arrival of hungry birds from the south, aphids, caterpillars, mosquitoes, grubs, flies and other insects of the north make their appearance just in time for the travelers to benefit by this nourishing food and, there is plenty of it to last a long time! The same is true of new plant life springing up, providing additional food and nesting materials.
But after several months in the north, the appetites of the growing bird families pretty well deplete this abundance of food. In contrast, southern lands have benefited by the birds' absence, and new supplies will be awaiting their return in the fall.
Incidentally, in the Arctic areas there has been a general warming of the atmosphere in the past 40 or 50 years, resulting in many birds now migrating there for summer months that did not go there before. Who told them about the improved climate? and how did they find the way there?