Migration is defined as the periodic mass movement of birds or other animals from one seasonal habitat to another, and back again. Migration may be partly carried out in response to climatic changes or changes in the food supply, and partly in relation to reproduction and the requirements of the young. Migration is an attempt to cope with the difficulties of a population that has outrun the immediately available means of subsistence. Over 4000 bird species migrate, most breeding in the northern latitudes of the hemisphere.
In the strict sense, migration is best illustrated by birds, many of which exhibit a mass movement from a breeding and nesting place to a feeding place and back again. In the northern hemisphere, migratory movements are exhibited by a large proportion of bird species, although the range differs greatly between species.
Birds have colonised most of the world, even islands that are deserted by other animals. This colonisation is only possible because birds can fly. Most harsh environments have relatively few bird species -only ones which have adaptations to the harsh conditions tend to inhabit such areas (eg. arctic areas, deserts, etc.) These birds can withstand the extremes of temperature and can find sufficient food to live permanently in such places. Seasonal changes can however result in a harsh environment becoming ideal for bird life.
The bird’s ability to move great distances quickly through flight has enabled it to take advantage of such seasonal changes, moving from area to area, avoiding a location when conditions are poor, and exploiting it when conditions are good. Birds that move backwards and forwards in a regular seasonal sequence are called migratory birds.
Migratory birds include:
- Those that move north or south in response to changes in temperature, and changes in the length of day and night.
- Those that move up and down mountains (called altitude migrants).
- Those that seasonally come to land to breed (eg. penguins).
Birds that move without any regular schedules, for example, desert birds responding to rainfall whenever it occurs, are called nomads.
Some of the remarkable feats of bird navigation still defy explanation. Most birds use a combination of environmental and innate cues to migrate. Natural selection also prunes back errors in migration, leaving only the best navigators to propagate the species.
The distance of migrations
Migrations can involve travelling short distances or long (across half the world). The Arctic tern flies 8,000 km from the Arctic to Antarctic, and back every year.
Such long distance migrants use their fat deposits as fuel, making stops to feed along their route. Many questions remain unanswered regarding bird migration. We do know that weather patterns can affect the particular time when a migratory flight starts, and how long the flight will last before the bird needs to rest and feed.
It is also known that birds use navigational aids such as:
- Star patterns
- The sun
- Topographical features they fly over
- The magnetic grid of the earth.
It seems that like so many other behavioural patterns in fish, mammals and birds, the urge and ability to migrate is innate, or instinctive, to all those who migrate.