You will hear a lecturer discussing how ideas about bird migration developed.
First, you have some time to look at questions 31 to 40.
Now listen carefully and answer questions 31 to 40.
Scientists now believe that most of the world's birds migrate in one way or another.
For some species, the seasonal journey is fairly short.
They might leave low ground where they spend the winter and move up into the mountains for summer.
At the other extreme is the Arctic tern, which travels over 25,000 miles each season between the regions around the North and South Poles.
For centuries, people have tried to understand these movements by observing birds.
Until quite recently, however, the winter destination of many birds remained a mystery.
Without modern scientific methods or technology, people proposed explanations that now seem mistaken and sometimes rather strange.
One old theory, dating back about 2,000 years, claimed that birds disappeared beneath seas and oceans to hibernate.
A related account suggested that they stayed hidden in mud through the winter and emerged when warmer weather brought food back.
The belief in hibernating birds survived for a surprisingly long time.
It was not finally rejected until the 1940s when experiments with captive birds showed that they have no instinct to hibernate.
Aristotle, the ancient Greek philosopher and naturalist, was among the earliest writers to consider why certain species vanished and returned at particular times of year.
From his observations of red starts and robins, he developed the theory of transmutation, the idea that one species became another as the seasons changed.
In autumn, he noticed red starts shedding their feathers.
He therefore concluded that they turned into robins for winter before changing back into red starts in summer.
Although that interpretation was wrong, it is easy to see why it appeared reasonable.
The two species are similar in shape.
It was a mistaken conclusion drawn from accurate observation.
An even more unusual proposal came from Charles Morton, an English amateur scientist of the 17th century.
In a paper that was taken more seriously than we might expect, he argued that birds traveled to the moon each year and later returned.
To him, this seemed the only rational explanation for the complete seasonal disappearance of some species.
A major advance in migration research occurred in 1822 when a white stork was shot in Germany.
The bird became famous because a long spear was lodged in its neck, yet had not killed it.
It was immediately clear that the weapon was not European, and it was later identified as belonging to a Central African tribe.
For ornithology, this discovery was decisive.
It provided the first evidence that storks spend the winter in sub-Saharan Africa.
The specimen, often called the arrow stork, can still be seen in the zoological collection at the University of Rostock in Germany.
Over time, people recognized the broad pattern of European birds traveling south in autumn and north in summer.
Even so, precise knowledge remained limited until researchers began catching birds and fitting rings to their legs.
Before ringing, almost nothing was known about the destinations reached by individual species or the routes they followed.
One suggestion was that large birds acted rather like taxis, carrying smaller birds on their backs.
Since some small species weigh only a few grams, many people thought they could not possibly cross vast oceans by themselves.
Observations of small birds harassing larger ones were taken as support for this belief.
Bird ringing was developed by the Danish schoolteacher Hans Christian Cornelius Mortensen, and it opened the way to many discoveries.
The method is still widely used and depends on what researchers call recovery.
A ringed bird is found dead at the place to which it migrated and can then be identified.
Enormous quantities of information were collected in the early 20th century, revealing for the first time where birds actually spent the winter.
In 1931, the first atlas showing the migration of common European species was published and more recent explanations of bird migration.
