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The Mystery of Migration
Mangrove and wetland wildlife at
Sungei Buloh Nature Park
Why do birds migrate?
The reasons are complex and not fully understood. But a simple explanation is food and a safe place to breed. Birds which breed in the summer in the extreme north such as the Arctic benefit from an abundance of food as plants and insect life flourish in the long daylight hours; and because few large permanent predators can survive the harsh winter. Many birds that breed in the Arctic simply lay their eggs on the ground. Being able to fly, they can avoid the harsh winter conditions, and be the first to arrive to enjoy the summer benefits.
In fact, some have suggested that the question should be why don't all birds migrate. Flight gives birds a huge advantage in finding new sources of food and good places to breed, that it is strange that not more birds migrate.
   
How did bird migration routes become established?
Migration is affected not only by food supply, but also by wind and oceans currents. These make some routes and locations easier to reach. While many birds migrate from northern breeding areas in the summer, to southern wintering grounds (mainly because there is more land near the northern pole than the southern), there are many other migration patterns. Some birds breed in the far south of South America, Australasia and Africa, and migrate to northern wintering grounds. Some birds migrate horizontally, to enjoy the milder coastal climates in winter. Other birds migrate in terms of altitude; moving higher up a mountain in summer, and wintering on the lowlands.
All kinds of birds migrate, from large cranes, birds of prey, to tiny hummingbirds. Even flightless birds migrate! Emus move from breeding sites in the rainy season to more permanent water sources in the dry. Penguins migrate in the ocean. Auk babies migrate by swimming until they fledge and can fly! Even birds that spend their entire non-breeding time in flight, such as seagulls, also move around on the ocean to follow seasonal food abundances.
   
How do birds migrate such long distances?
Birds exploit the winds to their favour so they can go the distance by burning minimal fuel. They may shift altitude to find the best wind "conveyor belt". Winds at high altitude may blow in the opposite direction from wind on the ground, and usually are blowing strongly. Larger birds rely on thermals (hot air) rising from the ground in the mornings to gain altitude by simply soaring. These birds usually migrate during the day. They may also follow strong updrafts along ridges.
The longest migration is undertaken by the Arctic Tern (Sterna paraisaea). It breeds in the Arctic North in the summer, then flies all the way to the other pole to spend winter on the Antarctic ice pack. The shortest distance between the two poles is 15,000km, but the birds usually travel a more circuitous route and can cover up to 20,000km; making a round trip of 30-40,000km!
A few birds fly non-stop, some for several days, covering enormous distances. But most birds break journey at staging posts. A vital aspect of being able to make such long trips is to lay down enough fat reserves. This is why staging posts such as Sungei Buloh are important to migrating birds.

Preparing for the journey: Besides laying down fat reserves, migrating birds also need to eat a lot to fuel their regular feather moults. Their feathers must be in tip-top condition for their long trips. Different species moult at different times; for most shorebirds it is just after breeding and before the migration to wintering grounds.

How do migrating birds find their way?
Studies suggest birds orientate themselves to the compass points using the position of the sun during the day, and the stars at night. They can also sense magnetic north. In addition they use other clues such as visual layout of the land, smell (of the sea), sound (waves on shores, winds through mountain passes).
The most amazing aspect of bird migration is that the location, route and perhaps even the techniques are hard-wired into their brains. Many migrating birds abandon their young as soon as they fledge, and a short time later, the young make the migration on their own.

Threats to migrating birds: Sadly, in addition to surviving storms and bad weather, exhaustion and other natural obstacles, migrating birds are increasingly face human threats. Habitat destruction that affects staging posts handicap their ability to re-fuel. These include draining wetlands, cutting down forests. Pollution of the sea, water and air also affects them. Migrating birds are also distracted and killed by lit-up skyscrapers, lighthouses and other unnatural man-made formations that mislead them. Sadly, many migrating birds are also hunted, for food, and for sport or superstitious reasons.

LINKS

  • BirdWatch Ireland: Migration-changing with the seasons: excellent explanation of migration with maps of migration in Europe and Africa; and excellent illustrations of how birds migrate plus details of experiments to highlight their talents; the effects of man on bird migration.
  • Northern Prairie Wildlife Research Center: Migration of Birds, based on the book by by Frederick C. Lincoln; Tons of details about bird migration in the Americas; origin and evolution, how scientists study bird migration, migration routes and patterns, and lots lots more.
  • The Why Files: lots of details about bird migration in the Americas; why and how they do it and threats to migratory birds.
  • Birding.About.com: lots of details about why and how birds migrate, with examples, details and maps about migration in the Americas.
  • Audubon Society: Bird Migration Facts with lots of details on why wetlands are crucial for migrating birds.
  • The National Aviary: a brief description of why birds migrate.
Websites on the East Asian Flyway

  • Scribbly Gum on the Australian Broadcasting Corporation website: links to waders and the East Asia Flyway

REFERENCES
  To buy these references & others, visit
Nature's Niche

  • Jonathan Elphick (ed.), "Collin Atlas of Bird Migration: Tracing the Great Journeys of the World's Birds", Harper Collins, 1995
  • David Attenborough, "The Life of Birds", Princeton University Press, 1998.
 
By Ria Tan, 2001