Single correct option
The best summary of the paragraph is required.
Many species of migratory bird navigate over thousands of kilometres with an accuracy that puzzled naturalists for centuries, since the birds require no learned landmarks and can orient correctly on their very first migration. Research over the past several decades has converged on an explanation involving multiple redundant senses: birds appear to detect the earth's magnetic field through specialised proteins sensitive to light in the eye, while also using the position of the sun, patterns of polarised light, and, in some species, olfactory cues, with different senses apparently serving as backups when others are unavailable, such as under heavy cloud cover.
Many species of migratory bird navigate over thousands of kilometres with an accuracy that puzzled naturalists for centuries, since the birds require no learned landmarks and can orient correctly on their very first migration. Research over the past several decades has converged on an explanation involving multiple redundant senses: birds appear to detect the earth's magnetic field through specialised proteins sensitive to light in the eye, while also using the position of the sun, patterns of polarised light, and, in some species, olfactory cues, with different senses apparently serving as backups when others are unavailable, such as under heavy cloud cover.
- A. Migratory birds rely solely on the earth's magnetic field to navigate, ignoring all other environmental cues.
- B. Bird migration remains entirely unexplained by science despite decades of dedicated research effort.
- C. Migratory birds achieve accurate long-distance navigation through several redundant senses, including magnetic, solar, and olfactory cues, which can substitute for one another when conditions limit any single one.
- D. Naturalists have proven that birds learn migration routes by following landmarks on their first journey.
Show answer and solution
Answer: (C) Migratory birds achieve accurate long-distance navigation through several redundant senses, including magnetic, solar, and olfactory cues, which can substitute for one another when conditions limit any single one.
The paragraph describes multiple senses (magnetic, solar/polarised-light, olfactory) working redundantly, with one substituting for another under poor conditions (e.g., cloud cover). Option 3 captures this multi-sense, redundant-backup explanation. Option 1 wrongly reduces this to a single sense. Option 2 contradicts the paragraph, which says the explanation has 'converged.' Option 4 is the opposite of what is stated—the birds need no learned landmarks.
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