The migration routes are without doubt the result of a long evolution and it is thought that birds may be gradually changing their routes today. A number of factors are involved in determining the routes which different species follow.
©1931 by Iowa State Teachers College
Clark, O. R.
"The Migration of Birds: Part II,"
Science Bulletin: Vol. 3:
7, Article 3.
Available at: https://scholarworks.uni.edu/science_bulletin/vol3/iss7/3