Monarch butterflies are renowned for their migration. Yet no single monarch has ever completed the 2,000 mile round trip – it is generational in nature because of the sheer distance involved. There being safety in numbers this leads to what is surely one of nature’s most spectacular sights – that of millions of monarchs congregating together.

In North America the butterfly begins this massive yearly migration in August. Monarch butterflies fuel up on nectar in the northern States and Canada - it is time to leave as the coming winter will be so cold it would inevitably kill them. Unlike their great-grandparents they have never flown more than a few hundred meters in their lives but they head out over vast northern lakes with no hesitation. This is the first leg of one of the world’s greatest migrations.Soon the single individuals are joined by other monarchs who have travelled from all over North America. The population east of the Rockies heads towards Mexico: the western population heads towards California, particularly the conifers of Santa Cruz and Pacific Grove. It is here that the spectacle occurs.When they get to their destination a lucky few humans will see countless butterflies hanging from every branch. They come here because although there will be a chill in the air it will not be lethal as it would be in the north. The conditions are perfect for hibernation or overwintering as it is also known.Although there is undoubtedly safety in numbers, the hibernating butterflies are vulnerable to predators despite the fact that monarchs are poisonous. Some birds have, however, learned to rip out the toxic parts and eat the rest. They kill hundreds of thousands of butterflies and dislodge many more. Yet that will hardly dint the population in the way that, say, deforestation has done.Those that fall from the branches must get back in to the trees as soon as they can – it is a matter of life or death. They vibrate their wings to warm their flight muscles to escape before the ground frost hits. Those that do survive huddle together in the trees for four months. The warmth of spring will awake them from their hibernation.The life span of a monarch is tiny by our standards. Those born in the early summer live for only two months. For the generation, the last of the summer, which will overwinter, nature provides a special form of deliverance. It is called the diapause and it is a non-reproductive phase which is a physiological state of dormancy which delays the aging process. It is the diapause which enables the monarch to overwinter.The majority of the butterflies survive and then they will take their first drink of nectar in four months. With the increasing warmth more and more awake and the sky is full of them when they alight from the trees. Soon they will all disperse northwards – their great grandchildren will return to escape the northern winter.The monarchs who have overwintered have now just a short life remaining. Before they stray too far from the site, however, the monarchs will find milkweed, on which their caterpillars thrive. There they will mate. Caterpillar and pupa stages last around two weeks apiece and once the new generation emerges they will continue their journey northwards.There they will lay eggs of their own. This will not be the generation which returns to the conifers of Mexico and California, however. That journey is to be made by the fourth generation in this cycle. Thus the monarchs have perpetuated themselves through millennia – and long may it continue!