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In May of 1822, 
Count Christian Ludwig von Bothmer

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shot down a stork over his castle
grounds in North Germany.

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However, he wasn’t the first person
to hunt that specific bird.

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Upon recovering the stork,

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von Bothmer found it impaled 
by a yard long wooden spear.

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A local professor determined 
the weapon was African in origin,

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suggesting that somehow, 
this stork was speared in Africa

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and then flew over 2,500 kilometers 
to the count’s castle.

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This astonishing flight wasn’t just
evidence of the stork’s resilience.

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It was an essential clue in a mystery
that plagued scientists for centuries:

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the seasonal disappearance of birds.

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Ancient naturalists had various theories
to explain the annual vanishing act

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we now know as migration.

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Aristotle himself proposed 
three particularly popular ideas.

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One theory was that birds transformed into
different bodies that suited the season.

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For example, summer time garden warblers

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were believed to transform 
into black caps every winter.

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In reality these are two distinct species—
similar in shape and size,

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but never appearing at the same time.

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Over the following centuries, birds 
were said to morph into humans,

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plants, and even the timbers of ships.

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This last transmutation was especially
popular with many Christian clergy.

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If barnacle geese were truly made of wood,

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they could be deemed vegetarian 
and enjoyed during meatless fasts.

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Aristotle’s second and even more enduring
hypothesis was that birds hibernate.

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This isn’t so far-fetched.

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Some species do enter short,

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deep sleeps which lower their heart rates
and metabolisms.

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And there’s at least one truly 
hibernating bird:

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the common poorwill sleeps out winters
in the deserts of North America.

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But researchers were proposing much more
outlandish forms of hibernation

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well into the 19th century.

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Barn swallows were said to remove 
their feathers and hibernate in holes,

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or sleep through the winter at the bottom
of lakes and rivers.

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Aristotle’s final theory 
was much more reasonable,

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and resembled something
like realistic migration.

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However, this idea was also taken 
to extremes.

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In 1666, the leading migration advocate
was convinced that each winter,

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birds flew to the moon.

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It might seem strange that prominent 
researchers considered such bizarre ideas.

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But to be fair, the true story 
of migration

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may be even harder
to believe than their wildest theories.

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Roughly 20% of all bird species 
migrate each year,

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following warm weather and fresh food
around the planet.

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For birds who spend their summers
in the northern hemisphere,

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this journey can span 
from 700 to over 17,000 kilometers,

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with some flights lasting as long
as four months.

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Birds who migrate across oceans may soar
without stopping for over 100 hours.

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Sleeping and eating on the fly, 
they navigate the endless ocean

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by the stars, wind currents, 
and Earth’s magnetic field.

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Tracking the specifics of these epic
expeditions is notoriously difficult.

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And while birds often take 
the most direct route possible,

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storms and human development
can alter their paths,

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further complicating our attempts
to chart migration.

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Fortunately, Count von Bothmer’s stork
offered physical proof

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not only that European storks were
migrating south for the winter,

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but also where they were migrating to.

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Ornithologists across the continent

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were eager to map the trajectory
of this flight,

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including Johannes Thienemann.

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Owner of the world’s first permanent
bird observatory,

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Thienemann was a major public advocate
for the study of birds.

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And to solve the field’s biggest mystery,

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he wrangled an army of volunteers
from across Germany.

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His team used aluminum rings to tag
the legs of two thousand storks

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with unique numbers and the address
of his offices.

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Then he advertised the initiative 
as widely as possible.

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His hope was that word of the experiment
would find its way to Africa,

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so people finding the tags would know
to mail them back with more information.

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Sure enough, from 1908 to 1913,

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Thienemann received 178 rings, 
48 of which had been found in Africa.

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Using this data, he plotted the first
migration route ever discovered,

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and definitively established that storks
were not, in fact, flying to the moon.