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  3. How Birds Find Way Home? The ability of birds to return to a familiar place from any distance is a remarkable feat of nature

How Birds Find Way Home? The ability of birds to return to a familiar place from any distance is a remarkable feat of nature

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问题                                      How Birds Find Way Home?
    The ability of birds to return to a familiar place from any distance is a remarkable feat of nature. For centuries people have taken advantage of this ability in homing pigeons by using them to take messages from distant points back to familiar sites. Homing pigeons are domesticated non-migratory birds with an instinct to return to their lofts (nesting sites) that is improved with training and by selective breeding. Training is started at short distances from the nesting site; over time, this distance is gradually increased to hundreds of miles from its loft at a completely unfamiliar location and it can fly in the direction of home within a minute or two of its release. How does this extraordinary behavior work?
    Understanding homing behavior is one of the greatest challenges to ornithologists (鸟类学家). Fortunately, because they are able to carefully control the conditions under which the pigeons are released, researchers have been able to learn a great deal about how the birds navigate their way home.
    Although homing ability has been fostered in pigeons by careful breeding and selecting of stock, it appears that training is not always necessary: Many species of wild birds perform similarly remarkable feats. One such bird is the migratory Manx Shearwater (剪嘴鸥). Built like tiny albatrosses, these seabirds spend most of their lives skimming over the ocean surface far from the sight of land. They come ashore only to nest in burrows, which they dig in the ground on offshore islands in order to be safe from predators. The ease of locating and observing their nests make shearwaters ideal subjects for homing experiments.
Great Bird Navigators
    Many migratory birds are remarkably faithful to previous nesting and overwintering places. Though a bird might be able to come close to these sites merely by flying in a general direction during the course of migration, at some point more sophisticated navigating techniques must take over to guide the bird to its precise destination.
    Many animals are able to find their way home. One way of doing this is to directly sense the goal—to see, hear, or smell it. Another way is to memorize the details of the outward journey and then reverse the route based on an integration of that information. Birds, however, apparently rely on a completely different process to find their way. To explain bird navigation, we have what is known as the "map-and-compass" theory.
    The compass component of this theory gives direction--north, south, east, west; the map component tells the bird where it is, or gives locality. Scientists have learned a great deal more about the compass component than they have about mapping. They know that birds have several means of determining compass directions, but unfortunately, they still have no satisfactory explanation for how birds use biological "maps" to guide them to a precise location from an unfamiliar starting point.
    Bird Sun Navigators
    Some observations indicate that birds might use the sun as, a visual cue to determine compass directions. Starlings (八哥), for example, seem able to negotiate the proper direction only if they have a view of the clear sky and sun; cloud cover seems to induce confusion. In an experiment in which the sun’s apparent position was changed with mirrors attached to an orientation cage containing starlings, observers noted that the direction of the starlings’ hopping, which earlier had been correlated to the direction that chose to migrate, was shifted accordingly.
    Even birds that migrate exclusively at night pay considerable attention to the sun. At first this may seem odd because, after all, the sun is not visible to the nocturnal (夜间活动的) birds when they are flying. On the other hand, it is a predominant feature in the sky at a time of day (dusk)when birds may well be making decisions about whether to fly that night and in what direction. Radar studies have shown that most night migrants take off during this twilight period.
    Like many other animals, birds are endowed with a built-in clock that tells them the time of day. Using this internal clock, young pigeons, at least, learn the sun’s path of movement across the sky. Many birds are known to have an internal clock, and many are known to have a sun compass, but it is only in pigeons that ornithologists can watch the learning process develop.
Bird Star Navigators
    Most birds migrate not by day, but at night. Employing conical (圆锥形的) cages, researchers quickly observed that night migrants exhibited hopping behavior similar to that of day migrants and that they oriented themselves in the proper direction under clear, starry skies but became disoriented when it was cloudy. The decisive tests were performed in a planetarium (天文馆), where star patterns can be controlled at will. The experiments indicated that night-migrating birds learn and orient by spatial relationships among the constellations (星座), rather than using information supplied by any single star. More recently, scientists have discovered that birds begin to develop star compass capability when they are quite young, and as experienced adults they can use many parts of the sky to recognize compass directions.
Magnetic Bird Navigators
    A sun compass for migration during the day, a star compass for nocturnal migration— life would be much simpler had this been the end of the story. For a long time there has been a popular theory that birds have a magnetic compass guiding their navigatory behavior. There is a good reason for that idea--it would be a convenient system to use during overcast days or nights. But many respected biologists assured ornithologists that for birds to sense such force was almost impossible because the Earth’s magnetic field is such a weak force. A group of German ornithologists conducted research that provided evidence to the contrary.
    In the study, night migrants that were placed in orientation cages indoors in closed rooms showed weak but consistent and seasonally appropriate hopping directions, which suggested that they did not need the sun or stars to determine direction. By placing the cages within sets of wire coils through which a weak electric current was passed, it was possible to change the configuration of the magnetic field surrounding the birds to determine if they respond in a predictable way. Although their responses continued to be weak, the birds did respond, and their orientation could be shifted by changing the magnetic directions. It therefore appears that in addition to sun and star compasses, at least several kinds of birds possess a magnetic compass, though as yet none of the field studies has found a clear indication of its influence on migratory birds in the wild.
Biological Bird Navigators
    After about three decades of experiments on homing pigeons, scientists currently have two viable hypotheses concerning the "mapping" ability of birds. Although only homing pigeons have been studied, there is good reason to believe that migratory birds also rely on some sort of biological map to find their way back to traditional nesting or wintering sites.
    The first hypothesis, conceived and tested primarily by a group of Italian scientists, involves an "odor map". The scientists propose that young pigeons learn this map by smelling different odors that reach their home loft on winds from varying directions. They would, for example, learn that a certain odor arrives on winds blowing from the east. If a pigeon is transported eastward from its loft it should smell that odor more strongly either on the way to or at the release site. This should tell the pigeon that it needs to fly westward to return home. Although it may sound absurd to some, there is a large amount of evidence supporting this hypothesis.
    The second hypothesis proposes that birds may be able to extract latitude and longitude from the Earth’s magnetic field. Unlike the compasses that are thought to help birds determine direction, this map is believed to help birds determine location. The main support for this hypothesis comes from observations of pigeons released in areas of "magnetic anomalies (异常)" (places where the Earth’s magnetic field is distributed due to large iron deposits near the surface). When pigeons are first released in these areas, they depart in random directions, but after their initial confusion, most birds are able to correct their course and return home once they escape the influence of the anomaly. Because, in theory, magnetic anomalies are not a strong enough force to affect any of the birds’ compasses—magnetic, sun, or star—proponents argue that the fact that the pigeons are initially affected indicates the existence of a different aspect of navigation that is being affected--hence, the map.
    Neither hypothesis has been proven to the satisfaction of all the experts, so new and different experiments continue to be performed. It may turn out that neither of these alternatives is correct, or a synthesis of the two may emerge.
The first hypothesis concerning birds’ "mapping,’ ability involves an"______".
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答案odor map
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