Millions of Red crabs start their migration on Christmas Island

Tens of millions of red crabs have begun their annual migration on Christmas Island, near Australia.

This spectacular yearly event sees around 50 million red crabs travel from their homes in the island forests to the ocean, to lay their eggs.

Special crab bridges have been built on the island to help the crabs get around tricky obstacles like roads, and the local radio stations provide people with an update on their movements.

Sir David Attenborough once described filming the crabs' migration as one of the most memorable moments in his career!

When does their migration start each year?

The red crabs annual migration starts with the first rainfall of the wet season.

On Christmas Island this is usually between October and November, but can sometimes be as late as

December or January.

The Moon also plays an important factor in when the crabs decide to migrate.

Red crabs will all leave their burrows at the same time when the Moon is in its third-quarter phase.

That's one heck of an internal calendar!

Why do they do this?

You might be wondering what causes millions of crabs to move from one end of the island to the other at the same time?

Well, the crabs are travelling to the ocean to mate and lay their eggs.

Once they've mated, the female crabs stay in little burrows on the beach for a few weeks whilst their eggs grow.

When the Moon reaches its final quarter and the tide is high, the female crabs will head into the water just before dawn to lay their eggs - which hatch straight away into tiny crab larvae.

Then, with that done, the crabs will head back home into the forests.