The blackpoll warbler weighs only three-quarters of an ounce, yet twice each year it flies 2400 miles [3862 km] non-stop for 4 days and nights. These little birds spend the summer in Alaska and then, in the fall, on one day they all know to begin flying eastward. Landing in eastern Canada or Northern New England, they eat as much as they can and wait some more. Then, suddenly, wherever they might be—though scattered from one another—they again take flight. In the air, they quickly become separated as they climb higher and higher in the sky. Although they want to go to South America, they begin by heading toward Africa. Climbing to 20,000 feet [6096 m] in the sky, they head off. How can each bird keep warm at such a high altitude? There is very little oxygen for it to breath, and it is so much harder to fly when its tiny wings must beat against that thin atmosphere. Yet on it goes, with nothing to guide it but trackless ocean below and sun, stars, and frequently overcast sky overhead. At a certain point, the little bird encounters a wind which does not blow at a lower altitude. It is blowing toward South America. Immediately, the little bird turns and goes in that direction. It had no maps, and no one ever instructed it as to the direction it should take. Well, you say, it may have taken the trip before. No, this might be one of this year's crop of birds which hatched only a few months before in Alaska. And its parents never told it what it was to do. Now, alone, separated from all the other birds, it keeps flying. It cannot stop to rest, eat, or drink. It dare not land on the water, for it will drown. Next spring, it will once again fly to Alaska.
Many other examples could be cited. One is a bronze bird in New Zealand which abandons its young and flies off. In March, when strong enough to fly, they follow after, taking the same route: first 1250 miles over open sea to northern Australia; then to Papua, New Guinea; then the grueling distance to the Bismarck Archipelago—a migration of 4000 miles from New Zealand where they hatched not long before.
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