The Red Crab migration is absolutely fascinating.
It All Depends on the Moon
The timing of the migration is linked to the phases of the moon and the lunar schedule.
The crabs mate at such a time that the females can produce and develop the eggs in burrows for about 10 - 12 days, before releasing them into the sea when high tide is between the last quarter and the new moon.
During this time, the sea level varies least for a longer period of time, making it easier and safer for the females to approach the edges for releasing their eggs.
If the migration gets delayed due to unfavorable weather conditions, spawning will be put off until the next lunar month.
