In May of 1822, 
Count Christian Ludwig von Bothmer

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

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

Upon recovering the stork,

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

A local professor determined 
the weapon was African in origin,

suggesting that somehow, 
this stork was speared in Africa

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

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

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

the seasonal disappearance of birds.

Ancient naturalists had various theories
to explain the annual vanishing act

we now know as migration.

Aristotle himself proposed 
three particularly popular ideas.

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

For example, summer time garden warblers

were believed to transform 
into black caps every winter.

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

but never appearing at the same time.

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

plants, and even the timbers of ships.

This last transmutation was especially
popular with many Christian clergy.

If barnacle geese were truly made of wood,

they could be deemed vegetarian 
and enjoyed during meatless fasts.

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

This isn’t so far-fetched.

Some species do enter short,

deep sleeps which lower their heart rates
and metabolisms.

And there’s at least one truly 
hibernating bird:

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

But researchers were proposing much more
outlandish forms of hibernation

well into the 19th century.

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

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

Aristotle’s final theory 
was much more reasonable,

and resembled something
like realistic migration.

However, this idea was also taken 
to extremes.

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

birds flew to the moon.

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

But to be fair, the true story 
of migration

may be even harder
to believe than their wildest theories.

Roughly 20% of all bird species 
migrate each year,

following warm weather and fresh food
around the planet.

For birds who spend their summers
in the northern hemisphere,

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

with some flights lasting as long
as four months.

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

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

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

Tracking the specifics of these epic
expeditions is notoriously difficult.

And while birds often take 
the most direct route possible,

storms and human development
can alter their paths,

further complicating our attempts
to chart migration.

Fortunately, Count von Bothmer’s stork
offered physical proof

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

but also where they were migrating to.

Ornithologists across the continent

were eager to map the trajectory
of this flight,

including Johannes Thienemann.

Owner of the world’s first permanent
bird observatory,

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

And to solve the field’s biggest mystery,

he wrangled an army of volunteers
from across Germany.

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

with unique numbers and the address
of his offices.

Then he advertised the initiative 
as widely as possible.

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

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

Sure enough, from 1908 to 1913,

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

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

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