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While we can only speculate about the reasons which initially lay at the root causes of the evolved habit of migration, we are on much sounder ground when we come to consider the conditions which, at the present moment, will cause a bird to migrate. These conditions are capable of experimental investigation, and some interesting and useful work has been carried out on this problem mainly in Canada and the United States. It has long been suggested that one of the main factors inducing the onset of the migratory urge may be sexual stimulation, brought about by the increasing amount of light to which birds are exposed as the days gradually get longer when the sun swings north again after the winter solstice. One experimenter sought to test this theory in an experimental aviary. He trapped a number of migratory birds (Canadian juncos) in the autumn as they were on their way south to winter quarters and placed them in an aviary lit by artificial means. When caught, the sex-organs of the birds were small and immature. He was able to show that, by manipulating the conditions of lighting, and exposing the birds to gradually increasing amounts of light, |
