閱讀前先看 Before you read
- 主題
- How migrating birds find their way
- 文章重點
- Migratory birds use several navigation systems together — a sun compass, a learned star compass and a magnetic compass — cross-checking between them rather than relying on any one alone.
- 難度
- 中等偏難 · Upper-Intermediate
- 建議時間
- 18 分鐘
重要單字
- migrate — to travel seasonally between regions / 遷徙
- compass — a system used to find direction / 羅盤
- compensate — to balance out, to make up for / 補償、調整
- planetarium — a room that projects an artificial night sky / 天文館
- innate — present from birth, not learned / 天生的
- cryptochrome — a light-sensitive protein in the retina / 隱花色素
30 秒快速理解 30-second summary
Birds make journeys of thousands of kilometres using multiple navigation systems. Kramer showed they use the sun and an internal clock; Emlen showed they learn a star compass from the rotating night sky; magnetite particles or cryptochrome proteins may let them detect Earth's magnetic field. No system works alone — birds cross-check between sun, stars, magnetism and landmarks.
逐段練習 Read paragraph by paragraph
1
段落 1 — The puzzle of long-distance navigation
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.
本段重要單字 (3)
- breeding — producing offspring / 繁殖
- featureless — having no distinctive marks / 沒有特徵的
- combination — two or more things working together / 結合
Quick Check · 隨堂小測
According to the writer, how do birds navigate?
- With one specialised navigation system.
- With several navigation systems used together.
- They cannot really navigate; they wander.
看答案 · Show answer
答案：B — With several navigation systems used together.
The final sentence says the answer involves "not one navigation system but several, used in combination".
2
段落 2 — The sun compass and the inner clock
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.
本段重要單字 (3)
- oriented — facing in a particular direction / 定向
- deflect — to change the angle or direction of / 偏轉
- compensate — to balance out a change / 補償、調整
Quick Check · 隨堂小測
Why must a sun-compass-using bird have an internal clock?
- Because the sun changes brightness throughout the day.
- Because the sun moves across the sky during the day.
- Because the sun disappears at night.
看答案 · Show answer
答案：B — Because the sun moves across the sky during the day.
The text states the bird "must compensate for the sun's daily movement across the sky — implying an internal clock".
3
段落 3 — A learned star compass
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.
本段重要單字 (3)
- celestial — relating to the sky or heavens / 天空的
- constellation — a named group of stars / 星座
- innate — present from birth / 天生的
Quick Check · 隨堂小測
What did Emlen's experiment show about the night-sky compass?
- Birds are born already knowing the celestial pole.
- Birds learn the pole of rotation during early life.
- Birds rely on a single bright star such as Betelgeuse.
看答案 · Show answer
答案：B — Birds learn the pole of rotation during early life.
Birds raised under a planetarium with a fake centre of rotation oriented to that fake centre, so the compass is "learned during early life, not innate".
4
段落 4 — Sensing the magnetic field
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.
本段重要單字 (3)
- magnetite — a magnetic mineral / 磁鐵礦
- cryptochrome — a light-sensitive protein in eyes / 隱花色素
- retina — the light-detecting layer at the back of the eye / 視網膜
Quick Check · 隨堂小測
How well is the magnetic-field receptor in birds understood?
- It is fully understood and uncontroversial.
- There are two leading candidates but it is still debated.
- Scientists agree it is in the upper beak only.
看答案 · Show answer
答案：B — There are two leading candidates but it is still debated.
The passage says "The receptor mechanism is still debated" and lists "two leading candidates" — magnetite and cryptochromes — so the science is not settled.
5
段落 5 — Cross-checking for accuracy
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.
本段重要單字 (3)
- redundancy — extra back-up to ensure reliability / 備援
- astonishing — extremely surprising / 驚人的
- navigator — someone who plans a route / 領航員
Quick Check · 隨堂小測
Why does the writer say birds cross-check several senses?
- Because no single sense is reliable enough on its own.
- Because most senses are completely useless during flight.
- Because human navigators told them to.
看答案 · Show answer
答案：B — Because most senses are completely useless during flight.
The opening sentence states "No single sense is fully reliable on its own"; the redundancy of multiple senses is what produces accurate journeys.
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