Write ONE WORD ONLY for each answer.

Bird Migration Theory

Most birds are believed to migrate seasonally.

Hibernation theory
• It was believed that birds hibernated underwater or buried themselves in [1].
• This theory was later disproved by experiments on caged birds.

Transmutation theory
• Aristotle believed birds changed from one species into another in summer and winter.
- In autumn he observed that redstarts experience the loss of [2] and thought they then turned into robins.
- Aristotle's assumptions were logical because the two species of birds had a similar [3].

17th century
• Charles Morton popularised the idea that birds fly to the [4] in winter.

Scientific developments
• In 1822, a stork was killed in Germany which had an African spear in its [5].
- previously there had been no [6] that storks migrate to Africa
• Little was known about the [7] and journeys of migrating birds until the practice of ringing was established.
- It was thought large birds carried small birds on some journeys because they were considered incapable of travelling across huge [8].
- Ringing depended on what is called the '[9]' of dead birds.
• In 1931, the first [10] to show the migration of European birds was printed.