A few years ago I wrote a series of four articles dealing with bird migration. It is a fascinating topic and I am going to run them again over the next few weeks. I have revised the articles to incorporate new information which is constantly being gained as a result of ongoing research.
Why do birds migrate instead of remaining in one suitably warm place all year long? Some scientists believe, that prior to the last ice age, when the northern hemisphere was much warmer than it is today, that many birds did not migrate at all. There was suitable habitat and climate even in parts of northern Canada.
Obviously, things changed drastically when much of North America became covered in a massive sheet of ice! The species that had previously lived in Canada, for example, would have been forced to move south. This was not a short stay since the ice covered the land for over three hundred thousand years, with a few relatively brief inter-glacial periods.
When the ice retreated, ten to twelve thousand years ago, new territory became available. Pioneering species moved out into this new land and adapted to its conditions. Some adapted so well that they were able to live in cold northern climates year round, species like chickadees, nuthatches and ravens, for example. Others found that the northern habitat was suitable in the summer months but not in winter, so they established a routine of moving back and forth with the changing seasons.
Generally, it is food supply, not temperatures that determines these birds’ movements. Some food items, evergreen cones for example, are available year round. Hence, birds that rely on such items do not need to migrate.
There is another reason that may explain why some birds migrate, that has nothing to do with glaciers. A swallow’s diet consists entirely of flying insects. Such food is available year round in the tropics, so why not just stay there all the time? Why the hazardous trip thousands of kilometres twice a year?
The answer may lie with a problem that is also facing the human species: over-crowding. As bird populations increased, the tropical regions could no longer feed all the birds, particularly during the breeding season when populations are swelled by the introduction of millions of young birds. So during the breeding season they spread out to take advantage of the seasonal abundance of food at the northern latitudes. But before this temporary food supply dries up in the winter, the adults, and their newly hatched young, must return south, where food is still available.