Bird migration is the movement of birds from the place where they breed to another area, then back again in time to breed. A migratory bird does this many times during its life. To escape the cold winter in Alaska, the eastern bar-tailed godwit flies 11,000 kilometres non-stop to New Zealand. The birds remain for about five months, and then fly back to Alaska (with a stopover in the Yellow Sea) to breed.
Migrating birds can be in one place that is good for nesting and raising chicks, then move to another area with a mild climate where there is plenty of food.
Birds that migrate to New Zealand
Many birds come from overseas to feed at estuaries and mudflats during New Zealand’s summer. Wading birds, such as lesser knots, bar-tailed godwits, whimbrels and eastern curlews, come from the northern hemisphere. Terns also arrive from the Arctic and from Asia.
Migrating from New Zealand
Only a few species that breed in New Zealand migrate overseas. Most shearwaters fly to other places in the Pacific. Some albatrosses travel to the seas around South America, and others go towards Australia. The long-tailed cuckoo and the shining cuckoo fly to the tropical Pacific.
Preparing for the journey
When the days get shorter as the seasons change, the birds put on extra weight so they will have energy for the long flight. Their stomach, liver and kidneys shrink, as these are not needed on the journey. Many birds grow bigger flight muscles.
Many birds stop to feed at places along the way. They use as little energy as possible by flying in formation, travelling in higher air that is less dense, or gliding rather than flapping their wings.
Finding their way
Some birds travel in groups led by experienced birds. Others migrate for the first time on their own. They use genetic programming that tells them where to go.
Birds may use the earth’s magnetism to help find their way. They can possibly sense this in the iron-rich crystals in their brains, or see magnetic fields as colours.
The position of the stars shows which way is north or south. The sun’s place in the sky at different times of day also indicates direction. Birds remember landmarks that they pass, sounds such as wind on ridges, and smells on the air to help them navigate.