Bird migration routes have been studied by a variety of techniques including the oldest, marking. Swans have been marked with a nick on the beak since about 1560 in England. Scientific ringing was pioneered by Hans Christian Cornelius Mortensen in 1899. Other techniques include radar and satellite tracking.
What is the oldest way to study migration? How have swans been marked? Who was scientific ringing pioneered by? Since when have swans been marked? What are other forms of studying migration?