The Earth’s magnetic field is also caused by convection currents deep within the planet, and indeed also flips from time to time — roughly every 200,000 years. While scientists have noticed a small decrease in the strength of the Earth’s poles, they shouldn’t be switching for another 2,000 years or so.
Birds of a feather land together
How flocking birds avoid colliding when they touch down
LANDINGS are the most perilous parts of flying. Airline pilots have to practise hundreds before they can carry passengers. Even then, they have co-pilots, air-traffic controllers and all sorts of gadgetry to help them. And they do it one plane at a time, on clearly marked runways. Now imagine swarms of aircraft all trying to land together on a small stretch of water with no assistance and no gizmos. The result would surely be disastrous. Waterfowl, however, frequently land in groups on featureless bodies of water, yet they rarely collide. So how do they manage it?
To find out, Hynek Burda, of the Czech University of Life Sciences in Prague, and his team of 11 zoologists armed themselves with maps, binoculars, compasses and anemometers. With these they observed the landings of nearly 15,000 birds of 14 species belonging to 3,338 flocks scattered across eight countries over the course of a year. The upshot of this ornithological marathon, published in Frontiers in Zoology, was a discovery remarkable in its simplicity: no matter from which direction a flock of birds approaches a body of standing water, its members usually land on it in alignment with the direction of the Earth’s magnetic field.
That birds have a magnetic sense is well known. It is, for example, one of the tools that allows long-distance migration. And Dr Burda’s suspicion that this sense may be involved in collision-avoidance explains why the team carried compasses. So the result was not a total surprise. But besides confirming their hypothesis, he and his colleagues also wanted to know exactly how birds do it.
The latest research suggests that birds detect magnetic fields in two ways. One relies on small pieces of magnetite (a magnetic iron oxide) lodged in their beaks, or inner ears, or both. The other employs a magnetism-sensitive chemical reaction in their eyes, allowing them to “see” the Earth’s magnetic field, probably as bright and dark spots superimposed on their visual fields, rather like the head-up display viewed by a fighter pilot. As a bird moves its head, the spots would shift position, allowing it to steer due north or south. More subtly, they might provide a reference independent of the local terrain from which to calibrate the optimal angle of descent. For this reason, Dr Burda suspected, the eyes would have it over the beak or the ears as the magnetic sense of choice during landing. He and his colleagues therefore looked at sequential photographs of 91 mallard ducks landing, and measured the angles of the birds’ heads relative to the horizon.
If collision-avoidance were based on normal visual cues, they reasoned, the ducks would sometimes look around to see where their neighbours were. Instead, every bird kept gazing forwards in exactly the same direction (due magnetic north or south), during all four phases of a landing: approaching with wings up; approaching with wings down to act as air-brakes; gearing (ornithologist-speak for popping out their landing gear, ie, their feet); and touchdown. But the ducks also held their heads at a constant angle to the horizon, which would not be necessary if they were merely using the spots to steer ahead. It would be important, though, if the spots also regulated their angle of approach.
The immobility of a mallard’s head when it is landing does not prove it is using its magnetic vision to steer, but it is certainly consistent with the idea. At the least, the birds’ agreement to land facing north or south makes collisions unlikely. If their magnetic sense also helps them all descend at the same angle, that would make the chance of them bumping into each other almost zero. How that angle is agreed on has yet to be determined. But even when it is, the most intrepid of human pilots are still unlikely to give it a try.