- [Narrator] In the heart of this grove, half a billion monarch butterflies lie sleeping.
They have traveled from as far as Canada to escape the cold northern winter.
How they return to this exact spot is a mystery.
Like the butterflies, hummingbirds feed on nectar, so our spy isn't seen as a threat.
While resting, they cluster together to conserve precious warmth.
But as the winter ends, the sun warms their fragile wings.
Once they reach a critical 50 degrees, they start to fly.
As the day warms, the forest comes to life.
(sparkling orchestral music) Spy humming bird won't harm these delicate butterflies.
Its moving parts are shielded to keep them safe.
Butterflies can even safely land on its wings.
It begins to capture something quite extraordinary.
What starts as a trickle soon becomes a stream.
Then grows into a cascade.
A spectacle that has rarely been filmed.
And our spy is harmlessly in the very heart of it.
(bright orchestral music) Beauty also lies in the detail.
(delicate orchestral music)