How do birds know when it’s time to migrate? There seems to be no single or simple answer to this question.
In the spring it is known that birds undergo certain physiological changes before migration. These changes are linked with the reproductive cycle.
Some birds, however, attain this “migratory state” some time before they begin their migratory flight. Some other, probably external stimulus must trigger the migratory urge.
In the fall, it’s not hard to identify possible contributing factors. Food supplies are diminishing: temperatures are dropping, and days are getting shorter. In the spring, as birds prepare to leave their warm tropical locales, it seems unlikely these factors make much difference to the birds. Neither food supply, nor temperature, nor length of day varies significantly in these areas.
Research has shown that birds are sensitive to a number of other external factors. These include changing star patterns, changing air pressure and humidity, changing wind patterns, and changes in the angle of the sun. It seems likely that some of these factors must act as cues telling the birds that “it’s time to go.”
Ornithologists are also looking closely at an apparent “internal clock” that birds seem to possess. Birds kept in laboratory conditions, away from all external stimuli, continue to exhibit physiological changes associated with breeding and migration. Over time, however, the clocks in these isolated birds run increasingly slower.
While most experts accept the existence of some instinctive sense of time, they agree that external stimuli are also required.
We tend to think of migration as being a north-south phenomenon. This, however, is not always the case. Some waterfowl, loons, swans, and grebes for example, move from inland breeding lakes to coastal areas in the winter. In various parts of the world this could involve a trip in almost any direction.
Here in B.C., a grebe might fly west from its breeding grounds in the interior, to winter in the salt water off the B.C. coast. In Europe, some birds spend summer in the deciduous forests of southern Europe and winter in the northern coniferous forests.
Some species exhibit a vertical migration, merely moving up and down mountainsides as the seasons change. The whole migration phenomenon is far more complicated than it first appears.