IELTS Reading 練習 鳥類如何遷徙（含題目+詳解）

How Birds Migrate

A. Twice a year, billions of birds make journeys of hundreds or thousands of kilometres between breeding grounds and wintering sites. Some, such as the Arctic tern, fly from pole to pole and back annually, covering more than 70,000 kilometres in the air. How birds find their way over such distances, often across featureless oceans and at night, is a question biologists have pursued for more than half a century. The answer turns out to involve not one navigation system but several, used in combination.

B. The earliest experimental work, conducted by the German biologist Gustav Kramer in the 1950s, showed that European starlings use the position of the sun as a compass. Kramer placed migrating birds in a circular cage open to the sky and observed that they oriented in the correct seasonal direction even with no other landmark visible. When mirrors were used to deflect the apparent angle of sunlight, the birds shifted their orientation by exactly the same amount. To use a sun compass usefully, however, a bird must compensate for the sun's daily movement across the sky — implying an internal clock — and Kramer's later experiments confirmed this.

C. A separate compass operates at night. In the 1960s, Stephen Emlen showed that European warblers raised under planetariums took their direction from the rotation of the apparent night sky around the celestial pole rather than from any single star. Birds reared under a sky in which the rotation centre had been moved to the constellation Betelgeuse oriented relative to that fictional pole when they later migrated. The mechanism is therefore learned during early life, not innate.

D. Most remarkable is the magnetic compass. Even in total darkness and without any visual cue, many migratory birds detect the direction of the Earth's magnetic field. The receptor mechanism is still debated, but two leading candidates are tiny magnetite particles in the upper beak and light-sensitive cryptochrome proteins in the retina that respond differently depending on field orientation. The latter would imply that birds, in some real sense, see the magnetic field.

E. No single sense is fully reliable on its own. Birds appear to cross-check between sun, stars, magnetic field and familiar landmarks, switching emphasis as conditions change — a redundancy that helps explain the astonishing accuracy of journeys that no human navigator with the same equipment would attempt.

The passage has five paragraphs, A-E. Choose the correct heading for paragraphs B, C, and D from the list below.

• i. A daytime compass that needs an internal clock
• ii. Combining several systems for safety
• iii. Reading the magnetic field, perhaps by sight
• iv. Distances and the question of method
• v. A learned compass for the night sky
• vi. A single, infallible sense

1. Paragraph B
2. Paragraph C
3. Paragraph D

1. Kramer found that starlings still oriented correctly when no landmarks were visible.
2. Emlen's planetarium experiments showed that the night-sky compass is innate.
3. The exact biological mechanism by which birds detect the magnetic field is now fully understood.

Complete the sentences below using NO MORE THAN TWO WORDS from the passage.

1. Birds raised under a planetarium oriented relative to the rotation of the night sky around the ______ pole.
2. One candidate magnetic receptor consists of tiny ______ particles in the upper beak.
3. To compensate for the sun's daily movement, a bird must rely on an internal ______.

1. i — A daytime compass that needs an internal clock Paragraph B is entirely about Kramer's sun compass and its requirement that "a bird must compensate for the sun's daily movement across the sky — implying an internal clock". Both halves of the heading match.

2. v — A learned compass for the night sky Paragraph C ends: "The mechanism is therefore learned during early life, not innate". Combined with the night-sky planetarium setup, "learned compass for the night sky" fits exactly.

3. iii — Reading the magnetic field, perhaps by sight Paragraph D introduces the magnetic compass and ends with cryptochromes that imply birds may "see the magnetic field". "Perhaps by sight" matches the hedging "in some real sense, see".

4. TRUE Supporting sentence: "the birds oriented in the correct seasonal direction even with no other landmark visible". Direct paraphrase.

5. FALSE Supporting sentence: "The mechanism is therefore learned during early life, not innate". The passage explicitly says learned, not innate. Direct contradiction.

6. FALSE Supporting sentence: "The receptor mechanism is still debated, but two leading candidates are ...". "Still debated" contradicts "fully understood".

7. celestial Supporting sentence: "took their direction from the rotation of the apparent night sky around the celestial pole". Single-word answer; the question supplies "pole".

8. magnetite Supporting sentence: "two leading candidates are tiny magnetite particles in the upper beak". Single-word answer.

9. clock Supporting sentence: "the bird must compensate for the sun's daily movement across the sky — implying an internal clock". Single-word answer; the question supplies "internal".