1Scientists believe that a majority of the earth's bird population migrate in some fashion or other.
2Some travel seasonally for relatively short distances,
3such as birds that move from their winter habitats in lowlands to mountain tops for the summers.
4Others, like the Arctic Tern,
5travel more than 25,000 miles seasonally between the northern and southern poles.
6Bird migration has been studied over many centuries through a variety of observations.
7But until relatively recently,
8where birds went to in the winter was considered something of a mystery.
9The lack of modern science and technology led to many theories that we now recognize as error-filled
10and even somewhat amusing.
11Take hibernation theory for example,
122000 years ago, it was commonly believed that when birds left an area,
13they went underwater to hibernate in the seas and oceans.
14Another theory for the regular appearance and disappearance of birds
15was that they spend winter hidden in mud till the weather changed and food became abundant again.
16The theory that some birds hibernate persisted until experiments were done on caged birds in the 1940s
17which demonstrated that birds have no hibernation instinct.
18One of the earliest naturalists and philosophers from ancient Greece was Aristotle
19who was the first writer to discuss the disappearance and reappearance of some bird species at certain times of year.
20He developed the theory of transmutation -
21the seasonal change of one species into another -
22by observing redstarts and robins.
23He observed that in the autumn,
24small birds called 'redstarts' began to lose their feathers,
25which convinced Aristotle that they changed into robins for the winter,
26and back into redstarts in the summer.
27These assumptions are understandable given that this pair of species are similar in shape,
28but are a classic example of an incorrect interpretation based on correct observations.
29The most bizarre theory was put forward by an English amateur scientist,
30Charles Morton, in the seventeenth century.
31He wrote a surprisingly well-regarded paper
32claiming that birds migrate to the moon and back every year.
33He came to this conclusion as the only logical explanation for the total disappearance of some species.
34One of the key moments in the development of migration theory came in 1822
35when a white stork was shot in Germany.
36This particular stork made history because of the long spear in its neck
37which incredibly had not killed it,
38everyone immediately realised this spear was definitely not European.
39It turned out to be a spear from a tribe in Central Africa.
40This was a truly defining moment in the history of ornithology
41because it was the first evidence that storks spend their winters in sub-Saharan Africa.
42You can still see the 'arrow stork' in the Zoological Collection of the University of Rostock in Germany.
43People gradually became aware that European birds moved south in autumn and north in summer
44but didn't know much about it until the practice of catching birds and putting rings on their legs became established.
45Before this, very little information was available about the actual destinations of particular species
46and how they travelled there.
47People speculated that larger birds provided a kind of taxi service for smaller birds
48by carrying them on their backs.
49This idea came about
50because it seemed impossible that small birds weighing only a few grams could fly over vast oceans.
51This idea was supported by observations of bird behaviour
52such as the harassment of larger birds by smaller birds.
53The development of bird ringing, by a Danish schoolteacher,
54Hans Christian Cornelius Mortensen,
55made many discoveries possible.
56This is still common practice today and relies upon what is known as 'recovery',
57this is when ringed birds are found dead in the place they have migrated to, and identified.
58Huge amounts of data were gathered in the early part of the twentieth century
59and for the first time in history people understood where birds actually went to in winter.
60In 1931, an atlas was published showing where the most common species of European birds migrated to.
61More recent theories about bird migration...