Multiple choice

Of the following descriptions of migrating animals, which most strongly suggests that the animals are depending on magnetic cues to orient themselves?

Directions: Answer the question based on the following passage.

It is ironic that many of us view tropical birds as strange, and perhaps even bizarre, when they vastly outnumber temperate-zone species. The paradox hits home when many aspects of birds we take for granted are, instead, products of a temperate-zone bias—due to the geographic distribution of people who study birds.

The capability of over-water migrants to travel on course is an occult phenomenon. Birds, bees and other species can keep trail of time without any sensory cues from the external world, and such “biological clocks” clearly contribute to their “compass sense.” To find north, for instance, they can use the position of the Sun or stars, along with the time of day, But compass sense alone cannot elucidate how birds navigate the ocean: after a flock traveling east is blown far south by a storm, it will presume the proper northeasterly course to recompense. Like any good pilot, the birds carry a compass. Recent research suggests that cells in the bird's head contain magnetite, an iron oxide crystal that aligns with magnetic north like a tiny compass needle. Scientists think these cells may serve as receptors that send directional information to the brain. Many other animals apparently also have such cells: Magnetite has been found in the heads of migratory fish, sea turtles and humpback whales. Of all the wildlife navigators, birds so far are the best studied. Perhaps, some scientists thought, migrants determine their geographic position on Earth by celestial navigation, almost as human navigators use stars and planets, but this would require of the animals a fantastic map sense.

Researchers now know that some species have a magnetic sense, which might allow migrants to determine their geographic location by detecting variations in the strength of the Earth’s magnetic field.

The researchers found that the birds' ability to select the correct feeder was significantly impaired if the birds had small magnets attached to their beaks. The scientists suggest this is because magnetite, an iron-rich crystal with magnetic properties, is found inside the birds' beaks.

  1. Pigeons can properly readjust their course even when flying long distances through exceedingly dense fogs.

  2. Bisons are able to reach their destination by passing through a landscape that has been partially alerted by a recent fire.

  3. Elephants are able to find grounds that some members of the herd have never seen before.

  4. Swallows are able to return to a given spot at the same time every year.

  5. Monarch butterflies coming from different parts of North America are able to arrive at the same location each winter.
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A Correct answer
Explanation

(1) (This is an 'inference' question, in which the key phrase is 'magnetic cues''). In first few lines, the author implies that, if only compass sense is being used by them for navigation over the ocean, a flock of birds traveling east will resume traveling east even after they have been blown far south by a storm, and not presume a northeasterly course to recompense, as they actually do. It is on this basis that he suggests that their navigation is based not on compass sense, but on magnetic sense.  We should therefore look for that statement among the choices which describes a situation in which a migrating animal readjusts its course after having been deflected from it through extraneous circumstances. (1) Itself describes such a situation. If pigeons depend only on a compass sense, and determine their migratory course with reference to only the sun and the stars, they would lose their way when passing through exceedingly dense fogs, because they will not be able to see the sun and the stars through such a fog. So, if they still readjust their course so as to be able to reach their intended destination, it means that they still readjust sense alone, but depend on magnetic cues. So, (1) is the answer. (2) is wrong because, if bisons migrate using their compass sense, based on the position of the sun and stars, a partial change in the landscape will not affect their movement. (3) Does not support either the compass sense theory or the magnetic sense theory, and is wrong. (4) And (5) can be elucidated even on the basis of the 'compass sense' theory, and do not point to the existence of a magnetic sense in the migratory birds and insects.

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