6 Migration

Key Idea: Migration is the long distance movement of individuals from one place to another. The risks and energetic costs of migration are offset by gains made at the destination. Migration is the long distance movement of individuals from one place to another. It usually occurs on a seasonal basis and for a specific purpose, e.g. feeding, breeding, or over-wintering. Many animals move great distances at

In order for migratory behaviour to evolve and be maintained, the advantages of migration must outweigh the disadvantages. Migration is a risky and energy expensive behaviour, and animals must spend a lot of time building energy stores that will fuel the effort. The destination provides enough food or shelter to enhance survival of individuals and their offspring. Although some animals migrate individually, many migrate in large groups (right and bottom). The energy cost of migration is different for birds, mammals, or other animals. Studies show that for a 100 g animal to travel 1 km a running mammal uses 2400 J, a flying bird uses 640 J and a swimming fish uses just 240 J. One gram of fat gained while feeding before a migration gives the animal about 37 kJ of energy, taking a mammal 15 km, a bird 54 km, and a fish 154 km. The migrations of walking or running animals therefore tend to be much shorter than those of flying or swimming animals. different times of the year or at certain stages in their life cycle. Migratory behaviour is innate but there may be a learned component for repeat migrants. The behaviour is triggered by an environmental cue, e.g. a change of season. Migration carries risk and has a high energetic cost. These disadvantages are offset by the benefits offered by the new environment, e.g. food supplies to support breeding.

Benefits of group migration

Group migration helps navigation by what is called the "many wrongs principle" in which the combining of many inaccurate navigational compasses produces a more accurate single compass. Thus, if an animal navigates by itself with a slightly inaccurate internal compass, or inaccurately interprets environmental cues, it may arrive in the wrong location. In a group, each member can adjust its heading according to the movement of the others, thus an average direction is produced and each member is more likely to arrive at the correct place (right). Increasing group size decreases the time taken to reach a navigational target when the group is moving as a social unit. Non-social groups take longer with increasing size because of the need to avoid others in the group (right). Flocking birds increase aerodynamic efficiency to each other, saving energy. In schooling fish, individuals in the centre of the school use less effort. Flocking and schooling also provide feeding benefits and reduce the risk of predation along the migration route.

1. What is migration?

Average time taken to reach target (arbitrary time units) Non-social movement No collision avoidance Group movement

Codling et al, Ecology, 2007

Group size

2. (a) What are the costs of migration to the migrant?

(b) How is the length of the migration related to energetic cost to the animal?

(c) Why do animals migrate despite these risks/costs?

3. (a) How does grouping together increase navigational efficiency?

(b) How does this enhance individual survival?