CAT 2021 Slot 1 VARC: How Birds Find Their Way

Each autumn, the Arctic tern departs its breeding grounds near the North Pole and flies, in stages, toward the edge of Antarctic sea ice, a round trip that can exceed seventy thousand kilometres in a year — the longest annual migration known among animals. That such a journey is possible at all, let alone repeated with enough precision that individual birds return to the same nesting sites, has made migration one of the most persistently studied phenomena in ornithology, and one whose mechanisms are still only partly understood.

Early explanations leaned heavily on a single faculty: birds were assumed to navigate primarily by sight, following coastlines, rivers and mountain ranges as a hiker might follow a marked trail. Landmark-following certainly plays a role, particularly for young birds on their first journey, but it cannot explain how migrants cross featureless oceans at night or maintain a heading through dense cloud. Subsequent research has revealed a layered system in which several independent senses are cross-checked against one another. Many species appear to detect the Earth's magnetic field, using a compass sense that may rely on light-sensitive proteins in the eye, allowing birds to perceive magnetic alignment as a visual pattern overlaid on their surroundings. Others calibrate direction using the position of the setting sun or the rotation of the night sky around the pole star, a skill young birds seem to learn by observing the stars' apparent movement during their first weeks of life rather than inheriting it outright.

More surprising still is evidence that some species use smell to construct a mental map of a home region, detecting minute differences in atmospheric odour that vary predictably with geography. This olfactory sense is thought to matter most in the final stages of a journey, when a bird must locate a specific nesting site rather than merely maintain a general heading — a task for which a magnetic compass, however accurate, is too coarse an instrument. The redundancy of these systems is itself informative: a bird relying on only one cue would be grounded by a single cloudy night or a local magnetic anomaly, whereas a bird that can fall back on smell, stars or a remembered coastline when one cue fails is far more likely to complete a journey spanning several continents.

None of this explains, however, why migration evolved in the first place, or why some populations of a single species migrate while others, living only a short distance away, do not. The leading account treats migration as a trade-off between the reproductive abundance of high-latitude summers — long days, fewer parasites, less competition for insect prey — and the punishing cost of winters there, so that the energetic expense of a long flight is repaid by the reproductive advantage gained at the destination. This framework is persuasive but incomplete: it predicts that migration should be favoured wherever the trade-off holds, yet closely related populations facing near-identical conditions sometimes differ in whether they migrate at all, suggesting that behavioural inheritance and historical accident, not environmental pressure alone, help decide which populations undertake the journey and which do not.