1 Introduction to movement and migration
Not all animals migrate in the same way as birds. The mass migrations of locusts do not involve a round trip for any individual, for their life cycle is shorter than the seasonal cycle. The round trip of the monarch butterflies (Danaus plexippus, Video 1) that migrate between North America and Mexico takes more than one generation to complete.
Download this video clip.Video player: Video 1
Transcript: Video 1 Monarch butterfly migration.
DAVID ATTENBOROUGH:
September on the shores of Lake Erie in southern Canada. A monarch butterfly is fuelling up on nectar. A chill gust of northerly air. It's time to leave. The coming winter will be so cold, it would kill her.
She has never flown more than a few hundred metres in her life. But now she is heading out over Lake Erie, which is a hundred miles across. This is just the first leg of one of the world's greatest animal migrations.
She will continue south, using the Sun as a compass to cross America, a journey of 2000 miles. Her destination is Mexico, and one small and special group of trees. No one knows how she finds them in these great mountain forests.
She joins other monarch butterflies that have travelled here from all over North America. Countless butterflies crowd these particular trees, hanging from every branch. They come here because, although winter brings a chill to the air, there will not be a lethal freeze as there will in Canada. The conditions are perfect for hibernation.
Hibernating butterflies are vulnerable to predators, but monarchs are poisonous. However, a few birds have learnt to rip out the toxic parts and eat the rest. They kill hundreds of thousands of butterflies and dislodge many more.
Those that fall must get back into the trees before nightfall brings another killer. They vibrate their wings to warm their flight muscles, but it's a race against time. Night brings a lethal ground frost. Those that do survive sleep safely huddled together in the trees for four months. The warmth of spring wakes them from their hibernation.
The majority of the butterflies that flew here from Canada have survived. They take their first drink since their arrival last autumn. With increasing warmth, more and more butterflies awake. Soon they will all disperse northwards and tranquillity will return to this forest until the great grandchildren of these butterflies return to escape the freezing northern winter.
Video 1 Monarch butterfly migration.
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The distances travelled during migration can be immense. Table 1 lists a few of the more spectacular journeys, and the animals that undertake them are shown in Figure 2.
| Record | Species | Distance |
|---|---|---|
| Longest recorded journey in water | leatherback turtle (Dermochelys coriacea) | 20 558 km in 647 days (Indonesia to Oregon, USA) |
| Longest mammal migration | humpback whale (Megaptera novaeangliae) | up to 8500 km each way |
| Longest insect migration | monarch butterfly (Danaus plexippus) | up to 4750 km in the autumn (North America to Mexico) |
| Longest recorded round-trip | arctic tern (Sterna paradisaea) | 70 900 km (North to South Pole) |
| Highest altitude migration | bar-headed goose (Anser indicus) | 9000 m above sea level |
