| Scientists believe that a majority of the earth's bird population migrate in some fashion or other. |
| Some travel seasonally for relatively short distances, |
| such as birds that move from their winter habitats in lowlands to mountain tops for the summers. |
| Others, like the Arctic Tern, |
| travel more than 25,000 miles seasonally between the northern and southern poles. |
| Bird migration has been studied over many centuries through a variety of observations. |
| But until relatively recently, |
| where birds went to in the winter was considered something of a mystery. |
| The lack of modern science and technology led to many theories that we now recognize as error-filled |
| and even somewhat amusing. |
| Take hibernation theory for example, |
| 2000 years ago, it was commonly believed that when birds left an area, |
| they went underwater to hibernate in the seas and oceans. |
| Another theory for the regular appearance and disappearance of birds |
| was that they spend winter hidden in mud till the weather changed and food became abundant again. |
| The theory that some birds hibernate persisted until experiments were done on caged birds in the 1940s |
| which demonstrated that birds have no hibernation instinct. |
| One of the earliest naturalists and philosophers from ancient Greece was Aristotle |
| who was the first writer to discuss the disappearance and reappearance of some bird species at certain times of year. |
| He developed the theory of transmutation - |
| the seasonal change of one species into another - |
| by observing redstarts and robins. |
| He observed that in the autumn, |
| small birds called 'redstarts' began to lose their feathers, |
| which convinced Aristotle that they changed into robins for the winter, |
| and back into redstarts in the summer. |
| These assumptions are understandable given that this pair of species are similar in shape, |
| but are a classic example of an incorrect interpretation based on correct observations. |
| The most bizarre theory was put forward by an English amateur scientist, |
| Charles Morton, in the seventeenth century. |
| He wrote a surprisingly well-regarded paper |
| claiming that birds migrate to the moon and back every year. |
| He came to this conclusion as the only logical explanation for the total disappearance of some species. |
| One of the key moments in the development of migration theory came in 1822 |
| when a white stork was shot in Germany. |
| This particular stork made history because of the long spear in its neck |
| which incredibly had not killed it, |
| everyone immediately realised this spear was definitely not European. |
| It turned out to be a spear from a tribe in Central Africa. |
| This was a truly defining moment in the history of ornithology |
| because it was the first evidence that storks spend their winters in sub-Saharan Africa. |
| You can still see the 'arrow stork' in the Zoological Collection of the University of Rostock in Germany. |
| People gradually became aware that European birds moved south in autumn and north in summer |
| but didn't know much about it until the practice of catching birds and putting rings on their legs became established. |
| Before this, very little information was available about the actual destinations of particular species |
| and how they travelled there. |
| People speculated that larger birds provided a kind of taxi service for smaller birds |
| by carrying them on their backs. |
| This idea came about |
| because it seemed impossible that small birds weighing only a few grams could fly over vast oceans. |
| This idea was supported by observations of bird behaviour |
| such as the harassment of larger birds by smaller birds. |
| The development of bird ringing, by a Danish schoolteacher, |
| Hans Christian Cornelius Mortensen, |
| made many discoveries possible. |
| This is still common practice today and relies upon what is known as 'recovery', |
| this is when ringed birds are found dead in the place they have migrated to, and identified. |
| Huge amounts of data were gathered in the early part of the twentieth century |
| and for the first time in history people understood where birds actually went to in winter. |
| In 1931, an atlas was published showing where the most common species of European birds migrated to. |
| More recent theories about bird migration... |