Animal 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).

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 (Table 1). Let’s take a closer look at the migrations undertaken by a bird, an insect and a mammal.

Figure 1: Bar-tailed Godwit feeding on a bivalve on northward migration at Yalu Jiang River, Yellow Sea, China

© 2010 Nature Education Courtesy of Phil Battley. All rights reserved.

Figure 2: Flight tracks of nine bar-tailed godwits on southward migration equipped with satellite transmitters

One female flew directly from Alaska to the non-breeding grounds in New Zealand on a 11,680 km non-stop journey across the Pacific Ocean, which lasted over 8 days (Gill et al. 2009).

© 2010 Nature Education Courtesy of US Geological Survey. All rights reserved.

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).

Figure 3: (A) Overwintering monarch butterflies gather at such high densities that the branches of their trees bend with their weight. (B) When the Southern right whales arrive on their feeding grounds around Antarctica in December they are often near starvation. But they are able to double their body weight after a short stay in the food-rich polar seas.

© 2010 Nature Education 3A courtesy of Raina Kumra; 3B courtesy of Courtesy of Michael Catanzariti. All rights reserved.

Migration is an adaptive response to the seasonal or geographic variation of resources (Gauthreaux 1982). The annual cycle of the seasons produces large differences in the duration and intensity of solar energy received in each hemisphere at any given time. Many migrants take advantage of favorable food and weather conditions offered in certain areas during very limited time periods. For instance, migratory birds like the bar-tailed godwit that breed at high latitudes (e.g., in the arctic tundra) exploit the extraordinary abundance of food during a few weeks in early summer and profit from long days, which allow them to extend foraging time. By leaving these areas after breeding, they avoid the northern winters with short days, low temperatures, and low food availability (Pulido 2007). In other cases, the resources needed in different life stages may be found in different locations. For example, young salmon migrate to sea to take advantage of the great abundance of food and high growth potential available there, but must someday return upstream to the small rocky tributaries required for spawning, a trip sometimes involving thousands of kilometres (Figure 4; Dingle 1996).

Figure 5: Migrating kokanee salmon Oncorhynchus nerka in Meadow Creek, British Columbia, Canada

© 2010 Nature Education Courtesy of Margaret A. Warren. All rights reserved.

The earliest evidence of humans being aware of animal migration dates back to the Stone Age. Some rock art images, portraying animals moving across the African savannah, are as old as 20,000 years. They were produced by nomadic hunter-gatherers, and could have served as a field guide to potential food, or as a visual record of good hunting areas.

The philosophers of Ancient Greece were the first to develop a theory of migratory behaviour in animals, even though their conclusions nowadays seem rather fanciful. Aristotle (384–322 BCE) realized that a number of birds were migratory, and his creative explanation for the sudden disappearance of summer migrants, such as warblers and swallows, was that they had morphed into different species present in winter. His idea of transmutation persisted well into the Middle Ages in Europe. According to another theory, which was still widely believed in the 1800s, migratory birds disappeared into the mud at the bottom of ponds and lakes where they would spend the winter. Since then, our understanding of migration has indeed improved.

Animal migration involves travelling from one type of habitat to another, which is often linked to the cycle of the seasons. Preparing for the migratory journey, which poses extreme energetic demands on the animal, usually involves a number of physiological changes, such as storing large amounts of fat and reducing the size of the organs that are not needed during migration. Migrants use a variety of cues to find their way, such as the magnetic field of the earth, changing concentration of minerals in the ocean water or polarized light. The breath-taking nature and raw beauty of animal migration has inspired humans for the past 20,000 years and continues to do so to this day.