When you think of animal migration, which animals first come to mind? Geese? Seabirds? Well, it’s true that many birds migrate during the year. In fact, at least 4,000 species of birds are considered regular migrants, which is equal to around 40% of all birds.
However, migration – the long distance movement on a seasonal basis – is actually displayed by all major animal groups. From the Monarch butterfly, that travels thousands of kilometres every autumn to hibernate in Mexico and Southern California; to the Humpback whale, that migrates between cooler and warmer waters to feed and breed.
For most species, migration comes at a great expense. Not only is there an enormous energy cost, but migratory animal also face the danger of predation, hunting, as well as unpredictable climatic conditions. So why run the risk?
In this article we’ll look at a selection of species, try to answer this question and start to understand this remarkable behaviour.
For Monarch butterflies, migration is a matter of life or death. During the summer months, they are found throughout North America but, from October, they fly almost 5,000 km southwards, where it is warm enough for them to survive the winter in hibernation.
Where they spend the winter depends on their initial distribution. If in the summer they live east of the Rocky Mountains, then they will migrate to Mexico. If they live on the western side of the Rocky Mountains, then they will hibernate in California. This is just one example of how geographical and geological features can influence the migratory route a species takes.
The Arctic tern migrates a longer distance than any other animal. They use a combination of adaptations, including magnetic orientation and magnetoreception, to find their way from one pole to the other and back again. That’s almost 80,000 km of travel in a year!
They fly north for the northern summer, to breed along the coastline of the Arctic Ocean, as well as Alaska and northern Canada. And they return south during the southern summer, where it spends most of its time at sea.
Turtle doves spend the summer months on their breeding grounds across Europe and the winter in Africa. By avian standards, there is nothing exceptional about the distance they travel. Nevertheless, because of hunting in southern Europe, their journey is a perilous one.
The European Commission estimates that, every year, around 2-4 million turtle doves are shot down over Malta, Cyprus, France, Italy, Spain and Greece. This, along with food shortages, disease and habitat loss, means that the turtle dove is now classified as vulnerable on the IUCN list of threatened species.
Humpback whale migration, involves a round trip of nearly 17,000 km, making it the longest migratory journey of any mammal. Every year they move between northern and southern hemispheres to reach their feeding and breeding sites.
Although Humpbacks can swim at around 5 mph, during a long migration they move much more slowly, resting and socialising along the way. Interestingly, not all members of a population arrive at their destination at the same time. Juvenile adults are the first to arrive, followed by mature males and then mothers and calves.
Migration, for the pacific salmon, is a matter of distance and difficulty. Having travelled downstream to the ocean, where they spend most of their adult life, after 2-6 years they then swim 3,000 km upstream through freshwater to spawn in the same stream they were born in.
This is an incredible feat of navigation, achieved thanks to a sensitivity to invisible olfactory cues. From the open ocean, salmon can find their natal river by detecting changes in concentrations of mineral salts in the water.
These are just a handful of the thousands of remarkable journeys different animals embark on every year. Much of the movement of animal over land, through the skies and across the oceans simply goes unseen. So it can be hard for us to imagine the distances and understand the scale of this global natural phenomenon.
In response to this challenge, Movebank have gathered migratory tracking data from 11,000 researchers for about 150 species to bring animal migration to life. Enjoy the video!