Many animals move from one place to another at certain times of the year. This _________ movement is called migration. They migrate to find food, _________ , or in search of warmer weather. One of the most _________ migrations in nature is that of the North American monarch butterfly.

Every autumn, millions of these beautiful insects with fine black and orange wings _________ a long and difficult journey. Somehow they manage to travel around 4,000 kilometres south and find their way to California or Mexico. However, until recently no one _________ how they did this.

A team of scientists led by Professor Eli Shlizerman at the University of Washington has now found the answer. They have found out that the monarch is able to _________ the time of day. It uses its eyes to measure the _________ of the sun. These two pieces of _________ — the time of day and the point where the sun is in the sky — allow the butterfly to _________ the way to go. ____________ , it manages to reach the places where it will spend the winter.

The ____________ to the mystery of the monarch’s amazing ability comes at a time when it is in serious trouble. Its population has ____________ by as much as 90 per cent in the last few years. Sadly, human ____________ is the main reason why the number of monarch butterflies is falling. In many of the places where the butterfly can be found, people are ____________ the natural environment. They ____________ trees and use chemicals that kill the plants that monarch caterpillars eat.

The ____________ on the monarch’s behaviour has however led to a greater awareness of this ____________ . People have been working together to ____________ its migration and make sure that there are enough plants for it to ____________ . If this works, there may come a time when the number of monarch butterflies increases once again, The more we know about this lovely creature, the greater the chance it will ____________ and keep its place in the natural world for a long time to come.