By “Awake!” correspondent in the British Isles
WHAT did Christopher Columbus, the Vikings, the Polynesian sailors and Noah have in common? Knowledge that the flight of certain birds can be used to indicate the presence of dry land.
Did you know that Columbus, on his first voyage across the Atlantic, altered course from west to southwest because many birds flew that way in the early evening? He discovered the Bahamas a few days later.
Birds are navigators par excellence. Consider just a few of their more amazing feats: The New Zealand bronze cuckoo is raised by ‘foster parents’ that do not migrate. Yet, these young birds, showing great navigational expertise, fly northward for 2,500 miles (4,000 km) across virtually empty ocean to the Solomon Islands. Great shearwaters are to be found as far north as the Shetland Islands, north of Scotland. However, when the breeding season approaches, they return by the millions to the Tristan da Cunha Islands, as far south as the southern tip of Africa. Even flightless penguins have this homing ability. Adélie penguins released 1,900 miles (3,000 km) from their home on the Antarctic ice returned there.
These journeys have all been carefully documented. A numbered band is placed around the leg of a bird, with a message requesting that anyone finding the bird communicate with the address on the band, giving the location of the creature when found. Although only a few are retrieved, the technique has enabled scientists to plot the flight paths of many migrating species.
In recent years birds have also been tracked by means of radar. Others have been fitted with tiny radio transmitters so that their flight paths can be followed. But the bulk of our knowledge has come from laboratory experiments. Before examining some of these, it would be enlightening to see what human navigators need in order to reach their destination.
Imagine a family going for a picnic. They park the car and hike into the woods to eat. In the evening, as they return, they lose their way and start walking in circles. What do they need in order to find the car? Two things—a map and a compass direction. The map must show them where the car is and where they are. By itself, however, this information will do them no good if they cannot orient themselves on the ground. They need a compass or something similar to point them in the right direction.
When we know a town well we do not have to carry maps and compasses with us, because we carry a map in our mind. Do birds carry a map in their brains? How do they find their compass bearings?
How Do Birds Navigate?
After much research, it has been well established that some birds can fly directly home when released in a strange area. This precludes the possibility that they find their way by first flying around in circles looking for familiar landmarks. These birds really can navigate. This involves more than simply flying south in the fall and north in spring on a normal migration flight. How they know where to fly is largely a mystery; in other words, the nature of any “map” that they may possess is unknown. Today, though, we do know of several systems that birds can use to orient themselves to fly in a constant direction.
Think of our family again. Suppose the father has a map in his pocket and uses it to find their position. He knows where their car is and realizes that they have to walk southeast to reach it. But how can he find southeast? Well, if there is a clear sky, he can find south by using a watch and the sun. How? He can hold the watch horizontally and point the hour hand toward the sun. The sun appears to move through about 15° per hour and there are 30° between any two consecutive hour divisions on a watch face. Thus a line midway between the hour hand and the “12” will point approximately south. Now it is a simple matter to find southeast. Can birds likewise use the sun to guide them?
In 1949 Gustav Kramer kept pigeons in drum-shaped cages, fitted with 12 identical food cups around the edge. He found that he could train the birds to eat from the cups pointing in a certain direction and that they used the sun for orientation. (In cloudy weather the birds ate from any food cup.) Thus pigeons gave evidence of having an internal clock that enabled them to compensate for the movement of the sun across the sky.
Kramer checked his results using starlings. He trained them to eat from certain cups, as before, but then substituted a moving light for the sun. Viewing this light as the sun, they ate from different cups, going from cup to cup at 15° per hour. In actuality, the light only moved up and down, and not horizontally, to mimic the sun’s rising and setting.
Many species of birds are now known to be able to fly in a constant direction quite accurately, using the sun and a built-in clock to help them. How accurate are their measurements? Well, an error of 1° in determining could put them approximately 70 miles (110 km) off course at the equator. A four-minute error in their clock would lead to the same great error. And birds have a reputation for pinpoint accuracy as navigators.
Let’s return once again to our family. If they wait until nightfall they will be able to use the stars as a compass that is more accurate than the sun. Can birds do the same? The answer seems to be yes. After all, many birds migrate only at night.
A German, Franz Sauer, in the 1950’s was the first to demonstrate the ability of birds to use the stars as a guide. He used blackcaps and garden warblers.
More recently, Stephen E. Emlen has experimented with North American indigo buntings. He placed the birds inside a planetarium, keeping them in cages designed to record the bird’s movements. When the time for their migration came around, they were shown a sky consistent with what they would see at that time of year outside. These birds showed a marked tendency to want to fly in what the planetarium told them was a southerly direction, their normal migrating course. Interestingly, the buntings seemed to recognize, not individual stars or constellations, but, rather, that the sky was rotating around a fixed point.
To test this, Emlen took buntings from nests, never letting them see the real sky. Instead of rotating the sky in the planetarium around the pole star, as is the real case, he adapted it to rotate around the star Betelgeuse. When the time came for them to migrate, these young buntings tried to fly away from Betelgeuse in what they evidently thought was a southerly direction.
Of course, much of the time the sky is overcast. While a hiking family would have no difficulty if they had a compass to direct them, how do birds fare in cloudy conditions?
Is Magnetism a Factor?
In 1885, A. von Middendorf proposed that birds could sense the earth’s magnetic field and find their way by means of it. This hypothesis was tested many times, with mainly negative results. It seemed inconceivable that a small bird such as a robin could detect magnetism. In recent years, however, evidence has been found showing that at least some species use the earth’s magnetic field for orientation. What is this evidence?
It was noted that many fast races between homing pigeons took place under a heavy cloud cover. So experimenters attached small magnets to individual pigeons that had demonstrated the ability to fly home in cloudy weather. They all lost their way. The pigeons were disoriented, evidently because the magnets disturbed the magnetic field around them. In another experiment, pigeons were fitted with frosted contact lenses. Although they could only see a few yards, a surprising number were able to fly within 200 yards (200 m) of their loft after a journey of 80 miles (130 km).
Others have experimented with migratory robins kept in cages. At the time for migrating they aligned themselves in the direction that they would normally be flying. Was the earth’s magnetic field guiding them? It would seem so, because the experimenters found that, by using electrical coils to alter the magnetic field, they could make the robins want to fly in another direction.
Scientists feel that perhaps the picture is still not complete. They are now studying how low-frequency sound, polarized light, smell, as well as changes in barometric pressures, could be used by birds to help them to navigate. Others are trying to ascertain the means by which some birds are able to detect magnetism.
Before the mystery is finally solved, there will no doubt be much more to surprise us in a field that has already produced many surprises.