IELTS Listening Practice cam 17 Test 03

Bird Migration Theory

Hibernation theory

• Most birds are believed to migrate seasonally.
• It was believed that birds hibernated underwater or buried themselves in 31.
• 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 32. and thought they then turned into robins.
• Aristotle's assumptions were logical because the two species of birds had a similar 33.

17th century

• Charles Morton popularised the idea that birds fly to the 34. in winter.

Scientific developments

• In 1822, a stork was killed in Germany which had an African spear in its 35.
• previously there had been no 36. that storks migrate to Africa
• Little was known about the 37. 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 38.
• Ringing depended on what is called the '39.' of dead birds.
• In 1931, the first 40. to show the migration of European birds was printed.