Question

Read the passage below and answer the question. The homing pigeon's sense of direction has puzzled scientists for centuries. A pigeon released hundreds of kilometres from its loft, in territory it has never seen, will usually find its way home within days. Experiments show that pigeons use the sun as a compass, correcting for its movement across the sky with an internal clock. On overcast days, when the sun is hidden, they switch to reading the Earth's magnetic field through tiny iron-rich structures in their beaks. Smell also plays a part: pigeons appear to build a mental map of odours carried on the wind, and birds with blocked nostrils often lose their way. No single sense explains the feat; the bird constantly cross-checks one system against another, falling back on a second or third cue whenever the first fails. Which of the following best states the central idea of the passage?