Reading 5 — how birds find their way

Sample question

Reading Passage — How birds find their way A Every autumn, billions of small birds leave their breeding grounds in Europe, Asia and North America and fly to wintering areas thousands of kilometres away, many of them travelling alone and at night. Some are making the journey for the first time, with no older bird to follow, and yet a large proportion arrive where their species has always arrived. How they manage it has been one of the longest-running puzzles in biology, and the answer has been assembled slowly, one experiment at a time, over the past seventy years. B The first clear evidence came from Gustav Kramer in Germany in the early 1950s. Kramer noticed that caged starlings became restless in the migration season and tended to flutter in the direction in which they would have flown. By using mirrors to shift the apparent position of the sun, he could shift the direction of that restlessness by a corresponding angle. The birds, he concluded, were using the sun as a compass. Because the sun moves across the sky during the day, such a compass is only useful to an animal that also knows the time, and Kramer's result implied that birds correct for the sun's movement with an internal clock. C Night migrants cannot use the sun. In the late 1960s Stephen Emlen, working in the United States, placed indigo buntings in a planetarium and found that their orientation followed the artificial stars: when the projected sky was rotated, the birds' heading rotated with it. More surprising was a later experiment with young birds raised under a sky that turned around a different star. When the migration season came, they oriented as if that star marked north. The birds, it seemed, were not born knowing any constellation. What they learned as nestlings was which part of the sky stays still while the rest turns around it. D A third cue does not depend on the sky at all. European robins kept in a closed room had been shown in the mid-1960s to orient using the Earth's magnetic field, and in 1972 Wolfgang and Roswitha Wiltschko showed that their compass behaved in an unexpected way. It did not respond to the polarity of the field — which end is north — as a hand-held compass does. It responded to the angle the field lines make with the ground, which distinguishes the direction of the nearer pole from the direction of the equator. Reversing the polarity of the field left the birds' choice unchanged; inverting its vertical part reversed it. E How a bird could sense such a field is still debated. In 1978 the physicist Klaus Schulten proposed that the answer lay in a chemical reaction, started by light, whose outcome can be influenced by very weak magnetic fields. The idea seemed exotic at the time, but it would explain why the robins' magnetic compass was later found to need light of certain wavelengths, and in the decades since, a light-sensitive protein in the bird's eye, cryptochrome, has become the leading candidate for the site of the reaction. If the theory is right, a bird may in some sense see the magnetic field as a pattern laid over its view of the world — though nobody knows what, if anything, that is like. F A compass, however, only gives direction. A pigeon released hundreds of kilometres from its loft in unfamiliar country must also know where it is. In Italy in the 1970s Floriano Papi and his colleagues found that pigeons deprived of their sense of smell often failed to return home from unfamiliar release sites, while their performance from familiar sites was much less affected. Papi argued that pigeons learn a map of the smells carried by the wind to their loft. The proposal was resisted for years, especially by researchers elsewhere whose own experiments produced weaker effects, and it remains the most disputed of the cues described here. G The most recent finding concerns people rather than birds. In 2014 a team led by Henrik Mouritsen at the University of Oldenburg reported that robins kept in wooden huts on the university campus could not orient magnetically at all — until the huts were lined with earthed aluminium screens that blocked the weak radio-frequency noise produced by electrical equipment. The noise was far below the levels set by safety guidelines for humans. Whether it affects migrating birds in the wild, rather than in huts, has not been established, but the result suggests that the magnetic sense is more delicate than anyone had assumed. H What emerges from these experiments is not one sense but a set of them, used together and checked against one another. Some studies suggest that migrants recalibrate their magnetic compass each evening against the glow of the setting sun, and a bird that cannot see the stars on a cloudy night is not lost if its magnetic sense is working. That redundancy may be the real answer to the old puzzle: no single cue is reliable everywhere, but it is rare for all of them to fail at once. — Which paragraph contains an example of an experiment in which changing one property of a cue made no difference to the birds?

• Paragraph D
• Paragraph B
• Paragraph E
• Paragraph G