This is clearly not the best place for butterflies to head for if they have a fear of crowds.
But congregating on trees in their millions and creating a wonderful orange glow as they fly around, the beautiful monarch butterflies at the Sierra Chincua reserve in Mexico are a spectacular sight.
The reserve hosts up to one billion butterflies each year, after they have made their migration from North America in a 2,500-mile journey to the warmer climes of Mexico between October and March.
The spectacular images were taken by photographer Joel Sartore, who was amazed by what he saw.
He said: ‘I was doing a story to document animal migrations in North America and covering this world famous event was a highpoint in the coverage
‘I loved the fact that so many of the butterflies congregate on the wintering grounds. It was like being in an orange blizzard at times.
‘It was a really wonderful life experience and one of the reasons I enjoy working as a photographer. It's a world class spectacle that's well worth seeing and an experience you'll never forget.’
When the butterflies migrate to the reserve they congregate thickly on trees to conserve heat, open their wings to catch the sun's warming rays and at dawn take flight in their millions and mate.
With a wing span of four inches, the monarch butterfly is known for its lengthy migration and is the only butterfly species to make annual north-south migrations, like many bird species.
The monarch migrates from North America to central Mexico and back again each year.
However, having a life span of only two months, it takes four generations to complete the round trip.
