动物迁徒，无论如何下定义，都远不只是动物群的移动而已。它可以大致被描述为按照规律的间隔——通常是以年度为循环周期——来进行的旅行，可能会涉及一个种群的许多成员，而且仅仅是在完成了长途跋涉之后才能获得回报。这种行为显示出了遗传的本能。生物学家Hugh Dingle总结出了五条，在不同程度上或以不同组合方式，适用于所有迁徙行为的特点。迁徙是旷日持久的长距离运动，将动物们带离熟悉的栖息地；它们往往是沿直线进行，而不是曲折迂回的；它们牵涉到一些与行前准备(例如超量进食)和到达有关的特殊行为；它们需要进行特殊的能量分配。并且还有一样：迁徙中的动物有着一种对更远大使命的格外专注，这使它们不会被任何诱惑转移了注意力，也不会因为任何会让其他动物望而却步的挑战而裹步不前。

Animal migration, however it is defined, is far more than just the movement of animals. It can loosely be described as travel that takes place at regular intervals - often in an annual cycle - that may involve many members of a species, and is rewarded only after a long journey. It suggests inherited instinct. The biologist Hugh Dingle has identified five characteristics that apply, in varying degrees and combinations, to all migrations. They are prolonged movements that carry animals outside familiar habitats; they tend to be linear, not zigzaggy; they involve special behaviours concerning preparation (such as overfeeding) and arrival; they demand special allocations of energy. And one more: migrating animals maintain an intense attentiveness to the greater mission, which keeps them undistracted by temptations and undeterred by challenges that would turn other animals aside.

一只北极燕鸥，在它从南美洲的最南端飞向北极圈的20,000公里途中，对于一个观鸟者从小船上提供给它的一条散发浓烈气味的美味鲱鱼将会毫不在意。本地海鸥将会贪婪地俯冲下来争食这般馈赠，而燕鸥却会继续向前飞去。为什么北极燕鸥之所以抗拒了这一分神因素，是因为那时那刻它被一种本能感觉所驱动着，我们人类发现这种感觉十分令人钦佩：它叫做“更远大的目标”。换言之，它下定决心一定要到达它的目的地。这只鸟感觉到它可以稍后再进食、休息和交配。当前它的注意力完全集中在旅程本身上；它的绝对唯一目的就是抵达目的地。去到北极的某个沙砾遍地的海岸，其他北极燕鸥都集结在了那里，这将让它达成那个由进化所塑造出来的更远大目标：找到某个地点、某个时间和一系列环境条件，它可以在其间成功地孵化和养育后代。

然而迁徙是个极其复杂的事件，而生物学家们对它的定义也各有不同，在某种程度上要取决于他们研究的是何种动物。蒙大纳大学的Joel Berger研究的是美洲叉角羚和其他大型陆生哺乳动物，他倾向于使用一个适用于他所研究动物类型的、被他称作简单实用的定义：“从某个季节性栖息区域去到另一个栖息区域然后再回来的往复运动”。这种季节性来回移动的原因通常是为了寻找某些在任何一个区域内都并非全年存在的资源。

An arctic tern, on its 20,000 km flight from the extreme south of South America to the Arctic circle, will take no notice of a nice smelly herring offered from a bird-watcher's boat along the way. While local gulls will dive voraciously for such handouts, the tern flies on. Why? The arctic tern resists distraction because it is driven at that moment by an instinctive sense of something we humans find admirable: larger purpose. In other words, it is determined to reach its destination. The bird senses that it can eat, rest and mate later. Right now it is totally focused on the journey; its undivided intent is arrival. Reaching some gravelly coastline in the Arctic, upon which other arctic terns have converged, will serve its larger purpose as shaped by evolution: finding a place, a time, and a set of circumstances in which it can successfully hatch and rear offspring.

但是海洋中浮游生物的每日垂直运动——夜里上浮以寻找食物，白天下潜以躲避捕食者——也可以被视作迁徒。蚜虫的活动也可被认为是迁徙：当一株食用植物上的所有嫩叶都被吃光以后，它们的后代就会飞去另一株宿主植物，没有任何一只蚜虫会回到自己出发的地方去。

Dingle是位研究昆虫的进化生物学家。他的定义比Berger的定义更为细致，列举出了将迁徙行为区别于其他形式动物活动的五条特征。它们考虑到了存在这样的事实情况，例如蚜虫在应该起身踏上它们大行程的时候会对蓝光(来自天空)变得敏感，而在应该下落的时候则对黄光(来自嫩叶的反射)敏感。鸟类在进行长途迁徙飞行之前会大量进食来为自身增脂。(也就是说，Dingle承认每个物种的迁徙行为都存在自身独特之处而彼此各有差异。)Dingle认为，他所下定义的价值在于，它集中关注了角马迁徙与蚜虫迁徙现象的共性，并以此来帮助研究者理解进化是如何导致所有这些共性的产生的。

But migration is a complex issue, and biologists define it differently, depending in part on what sorts of animals they study. Joel Berger, of the University of Montana, who works on the American pronghorn and other large terrestrial mammals, prefers what he calls a simple, practical definition suited to his beasts: 'movements from a seasonal home area away to another home area and back again'. Generally the reason for such seasonal back-and-forth movement is to seek resources that aren't available within a single area year-round.

然而，人类活动正在对动物迁徙产生着有害影响。叉角羚虽然看起来颇似羚羊，但其实二者并无关系，它是新世界(注：New World 是英国人对美洲大陆的旧称；相应地，英美对传统欧洲国家则称之为Old World)里速度最快的陆生哺乳动物。其中一个种群会在美国西部大提顿国家公园的山脉间度过夏天，然后从其山间的夏季牧场沿一条狭窄路径南下，穿过一条河，最后来到平原上。它们在这里熬过最寒冷的几个月，主要靠吃被风吹露出雪面以上的灌木蒿丛度日。这些叉角羚之所以引人注目，在于它们迁徙路线的年复一年从不改变，并且这条路线在三个瓶颈隘口，狭窄难行。如果它们在春季迁徙的过程中不能通过这三个路口中的任何一个，就无法抵达它们水草丰美的夏季乐园；如果它们在秋季再次穿行的时候不能通过路口而向南躲避到这些有风吹袭的平原上，它们就有可能在北方厚厚的雪层中试图过冬而死亡。叉角羚依靠远视能力和奔跑速度来躲避捕食者，一般穿行于平原的开阔凸起地带，在这样的地方它们才能四下张望和撒蹄狂奔。在这些隘口中的一处，两侧林木覆盖的山峦耸立构成了一个V形，留出一条只有大约150米宽的走廊空地，其上还建满了私人住宅。不断的发展正在引发一场叉角羚的生存危机，眼看就要封住了它们的穿越通道。

物种保护科学家们，以及来自美国国家公园管理局和其他机构的一些生物学家和土地管理者们，现在正致力于保护动物的迁徙行为，而不是仅着眼于物种和栖息地的保护。己有一片国家森林将叉角羚的迁徙路径，其中一大部分路程要穿越该森林内部，列为一条受保护的迁徙走廊。但无论森林保护局还是公园保护局都无法操控某个狭窄口的私人土地上到底会发生什么。而且由于另一些物种也会进行迁徙活动，这一挑战变得更加复杂，因为这些影响因素：动物长途跋涉走过的遥远路途、更多的土地管辖权、更多的边境、沿途的更多危险。我们将需要智慧与决心来确保这些迁徙的物种还能再将它们的长途行走活动进行得更长久一些。

But daily vertical movements by zooplankton in the ocean - upward by night to seek food, downward by day to escape predators - can also be considered migration. So can the movement of aphids when, having depleted the young leaves on one food plant, their offspring then fly onward to a different host plant, with no one aphid ever returning to where it started.

Dingle is an evolutionary biologist who studies insects. His definition is more intricate than Berger's, citing those five features that distinguish migration from other forms of movement. They allow for the fact that, for example, aphids will become sensitive to blue light (from the sky) when it's time for takeoff on their big journey, and sensitive to yellow light (reflected from tender young leaves) when it's appropriate to land. Birds will fatten themselves with heavy feeding in advance of a long migrational flight. The value of his definition, Dingle argues, is that it focuses attention on what the phenomenon of wildebeest migration shares with the phenomenon of the aphids, and therefore helps guide researchers towards understanding how evolution has produced them all.

Human behaviour, however, is having a detrimental impact on animal migration. The pronghorn, which resembles an antelope, though they are unrelated, is the fastest land mammal of the New World. One population, which spends the summer in the mountainous Grand Teton National Park of the western USA, follows a narrow route from its summer range in the mountains, across a river, and down onto the plains. Here they wait out the frozen months, feeding mainly on sagebrush blown clear of snow. These pronghorn are notable for the invariance of their migration route and the severity of its constriction at three bottlenecks. If they can't pass through each of the three during their spring migration, they can't reach their bounty of summer grazing; if they can't pass through again in autumn, escaping south onto those windblown plains, they are likely to die trying to overwinter in the deep snow. Pronghorn, dependent on distance vision and speed to keep safe from predators, traverse high, open shoulders of land, where they can see and run. At one of the bottlenecks, forested hills rise to form a V, leaving a corridor of open ground only about 150 metres wide, filled with private homes. Increasing development is leading toward a crisis for the pronghorn threatening to choke off their passageway.

Conservation scientists, along with some biologists and land managers within the USA's National Park Service and other agencies, are now working to preserve migrational behaviours, not just species and habitats. A National Forest has recognised the path of the pronghorn, much of which passes across its land, as a protected migration corridor. But neither the Forest Service nor the Park Service can control what happens on private land at a bottleneck. And with certain other migrating species, the challenge is complicated further - by vastly greater distances traversed, more jurisdictions, more borders, more dangers along the way. We will require wisdom and resoluteness to ensure that migrating species can continue their journeying a while longer.