To South Americans, robins are birds that fly northevery spring. To North Americans, the robins simply vacation in the south eachwinter. Furthermore, they fly to very specific places in South America and willoften come back to the same trees in North American yards the following spring.The question is not why they would leave the cold of winter so much as how theyfind their way around. The question perplexed people for years, until, in the1950s, a German scientist named Gustave Kramer provided some answers and, inthe process, raised new questions.
Kramer initiated important new kinds of researchregarding how animals orient and navigate. Orientation is simply facing in theright direction; navigation involves finding ones way from point A to point B.
Early in his research, Kramer found that caged migratorybirds became very restless at about the time they would normally have begunmigration in the wild. Furthermore, he noticed that as they fluttered around inthe cage, they often launched themselves in the direction of their normalmigratory route. He then set up experiments with caged starlings and found thattheir orientation was, in fact, in the proper migratory direction except whenthe sky was overcast, at which times there was no clear direction to theirrestless movements. Kramer surmised, therefore, that they were orientingaccording to the position of the Sun. To test this idea, he blocked their viewof the Sun and used mirrors to change its apparent position. He found thatunder these circumstances, the birds oriented with respect to the new"Sun." They seemed to be using the Sun as a compass to determinedirection. At the time, this idea seemed preposterous. How could a birdnavigate by the Sun when some of us lose our way with road maps? Obviously,more testing was in order.
So, in another set of experiments, Kramer put identicalfood boxes around the cage, with food in only one of the boxes. The boxes werestationary, and the one containing food was always at the same point of thecompass. However, its position with respect to the surroundings could bechanged by revolving either the inner cage containing the birds or the outerwalls, which served as the background. As long as the birds could see the Sun,no matter how their surroundings were altered, they went directly to thecorrect food box. Whether the box appeared in front of the right wall or theleft wall, they showed no signs of confusion. On overcast days, however, thebirds were disoriented and had trouble locating their food box.
In experimenting with artificial suns, Kramer madeanother interesting discovery. If the artificial Sun remained stationary, thebirds would shift their direction with respect to it at a rate of about 15degrees per hour, the Sun's rate of movement across the sky. Apparently, thebirds were assuming that the "Sun" they saw was moving at that rate.When the real Sun was visible, however, the birds maintained a constant directionas it moved across the sky. In other words, they were able to compensate forthe Sun's movement. This meant that some sort of biological clock wasoperating-and a very precise clock at that.
What about birds that migrate at night? Perhaps theynavigate by the night sky. To test the idea, caged night-migrating birds wereplaced on the floor of a planetarium during their migratory period. Aplanetarium is essentially a theater with a domelike ceiling onto which a nightsky can be projected for any night of the year. When the planetarium skymatched the sky outside, the birds fluttered in the direction of their normalmigration. But when the dome was rotated, the birds changed their direction tomatch the artificial sky. The results clearly indicated that the birds wereorienting according to the stars.
There is accumulating evidence indicating that birdsnavigate by using a wide variety of environmental cues. Other areas underinvestigation include magnetism, landmarks, coastlines, sonar, and even smells.The studies are complicated by the fact that the data are sometimescontradictory and the mechanisms apparently change from time to time.Furthermore, one sensory ability may back up another.
参考译文：定位和导航
在南美，知更鸟每一年都会飞往春天时的北方。对于北美而言，知更鸟每年冬天又都会在南美度过简单的“假期”。甚至，它们会飞往南美几个特定的地方，然后在第二年春年又飞回到北美相同的树界范围内。问题是它们为什么会在寒冷的冬天离开，然后又是怎样找到迁徙的路径的。这个问题困扰了人们很久，直到1950年代，一位名叫Gustave Kramer 的德国科学家给出了一些回答，意想不到地又提出新的问题。
就动物如何定位和航行的问题，Kramer发起了意义重大的新类型的研究。定位仅仅就是朝向正确的方向，而航行还涉及寻找从点A到点B的路径。
在研究早期，Kramer发现被关在笼子里的侯鸟同往常去野外开始迁徙的时候变得焦躁不安。而且，他注意到，当这些鸟在笼子里躁动不安时，它们通常会飞向迁徙路径的方向。于是，Kramer用星椋鸟做实验，将它们关在笼子里，总结出了它们的迁徙方向。事实上，它们基本都能朝向正确的迁徙方向，阴天除外。因为阴天的时候它们的骚动不安使得它们难以清楚分辨方向。因此，Kramer推测，星椋鸟是通过太阳方位来确定方向的。为了证实这一推测，他将这些鸟的眼睛蒙住，并且用镜子改变太阳的自然方位。他发现，在这种环境下，这些鸟会依照新的“太阳”来定位。似乎它们把太阳作为一个罗盘来决定它们的方向。当时人们认为这种观点非常荒谬可笑，有些人在有地图的情况下都有可能迷路，鸟儿又怎么能够用太阳进行导航呢？很显然，接下来还需要做更多的实验。
因此，Kramer又做了外一组试验，他在鸟笼周围摆放上相同的鸟食罐，但是只有一个食罐中有食物。所有食罐的位置都是固定的，而且装有食物的那个食罐始终置于罗盘的同一个方位。但是，这个位置会随周围环境而发生变化，转动关着鸟的笼子或者背景墙都会使得这个食罐的位置相对改变。可是，只要这些鸟能够看见太阳，不管周围环境如何变化，它们都能立即找到那个装有食物的食罐。不论这些食罐是在右侧还是左侧墙壁前方，它们都没有表现出一丝疑惑。可是阴天的时候，它们就无法定位方向，很难找到装有食物的食罐。
在关于人工太阳的试验中，Kramer还有一些很有意思的发现。如果人工的太阳位置保持不变，这些星椋鸟会以每小时15°角的速度改变它们的方向，而这一速度正好是太阳在天空中运行的速度。显然，这些鸟认为它们看见的“太阳”是按照这个速度移动的。但是，当它们看见真正的太阳时，却保持了恒定的方向，正如太阳在天空中移动一样。也就是说，它们可以适应太阳的运行。这就意味着，它们形成了非常精准的生物钟。
那些在夜间迁徙的侯鸟又是怎样的呢？也许它们通过观察夜晚的天空来定向飞行。为了证实这一推测，把在夜间迁徙的侯鸟关进笼子里，并在它们的迁徙的时间段，将笼子置于一个天文馆的地板上。这个天文馆实际上是一个剧场，天花板呈穹顶状可以投射出一年四季所有夜晚的景象。当天文馆的穹顶与外面的天空相吻合时，这些鸟就会朝着往常迁徙的方向拍打着翅膀。但是当穹顶旋转时，这些鸟就会改变方向以适应这个人造天空。这就清楚地表明这些夜间迁徙的侯鸟是通过星宿位置来定位方向。
这些不断积累的证据表明鸟是通过广泛多样的外界环境信息来引导它们迁徙的。而包括磁场、地标、海岸线、声波甚至气味也被作为实验对象进行观察。由于这些数据有时会自相矛盾并且物理过程经常随着时间发生变化，使得这些研究非常的复杂。此外，一种感知能力可能会支持另一种。
星椋（liang）鸟，羽毛蓝色，有光泽，带乳白色斑点，嘴小带黄色，眼靠近嘴根，性好温暖，常群居，吃植物的果实或种子．
(选自TPO11)
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