Animal migration has fascinated humans for millennia. And for good reason: Some of the most astonishing feats of endurance are performed by long-distance migrants, such as the nine-day non-stop flight of the bar-tailed godwit across the Pacific Ocean.
What is Migration?
Most people think of migration as the seasonal movement of a flock of birds between their breeding and non-breeding sites. In fact, bird migration is probably the biological phenomenon that has attracted the most interest among non-scientists, and has one of the longest traditions of scientific investigation in biology (Berthold 2001). However, there are many other forms of animal migration, including journeys between east and west, complex round-trips involving land and ocean, altitudinal journeys up and down mountains, and vertical movements through the water column of oceans and lakes (Hoare 2009). What sets migration apart from other forms of movements is that migration typically involves travelling from one type of habitat to another (Aidley 1981).
Who Migrates?
Migrating animals are found in all major branches of the animal kingdom. They include taxa as diverse as fish, crustaceans, amphibians, reptiles, insects, mammals and slime moulds (Bowlin et al. 2010). The distance covered during some of the journeys, and the altitude at which they can take place, are simply astonishing. Let’s take a closer look at the migrations undertaken by a bird, an insect and a mammal.
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Table 1. Guinness records held by animal migrants |
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Smallest migrant |
1–2 mm long |
Zooplankton (crab larvae, copepods, etc.) |
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Largest migrant |
24–27 m long |
Blue whale (Balaenoptera musculus) |
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Longest mammal migration |
Up to 8,500 km each way |
Humpback whale (Megaptera novaeangliae) |
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Longest insect migration |
Up to 4,750 km in the autumn |
Monarch butterfly (Danaus plexippus) |
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Longest recorded round-trip |
80,000 km |
Artic tern (Sterna paradisaea) |
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Highest migration altitude |
9,000 asl |
Bar-headed goose (Anser indicus) |
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Source: Hoare 2009, Egevang et al. 2010 |
One of the longest non-stop flights is performed by the bar-tailed godwit (
Limosa lapponica). The
L. l. baueri subspecies breeds in Alaska and migrates south to its non-breeding quarters in Australia and New Zealand. Tracking individuals using lightweight satellite tags revealed that these birds are able to cross the Pacific Ocean in a non-stop marathon flight lasting over 9 days, making it one of the most impressive feats among migratory animals.
Perhaps the best-known insect migrant is the monarch butterfly (
Danaus plexippus). At the end of every summer, more than 100 million individuals sweep across North America to overwinter up to 4,750 km further south in California and Mexico. The complete migration requires an intergenerational relay. No single individual makes the entire round trip, as the duration of these journeys exceeds their lifespan. Instead, the females lay eggs during the migration, from which the next generation of migrants emerges. Monarchs specialize on milkweeds (Asclepiadaceae), which contain substances that are poisonous to vertebrates and thus protect the monarch from many potential enemies (Dingle 1996).
The common name of the southern right whale (
Eubalaena australis) (Figure 3b) refers to the fact that they were considered the ‘right’ whale to hunt. They are slow-moving and have large amounts of blubber, which meant that they floated after being killed. Many tens of thousands were slaughtered until the international whaling ban in 1935. Southern right whales migrate from their Antarctic feeding areas to temperate breeding areas along the costs of Chile and Argentina, southern Africa, and Australia and New Zealand, covering 2,500 km each way. Their migration is fuelled entirely by fat accumulated during their four-month stay in the icy Southern Ocean around Antarctica, where they skim the surface waters for zooplankton. Amazingly, they will not feed until their return a year later (Hoare 2009).