The sudden appearance and disappearance of large numbers of birds as the seasons change has mystified people around the world for thousands of years. Aristotle imagined that some of the vanished birds were hibernating and that others were transformed from one species into another. An essay written in 1703 claimed that the departing birds flew to the moon.
Māori also speculated. They observed the seasonal arrivals and departures and noted that no one ever found the nests or chicks of certain birds such as the godwit (kūaka). It was believed that they returned to the spiritual homeland of Hawaiki to breed.
We now know more about where these birds go. We can compare notes with observers in regions where a species appears after vanishing from New Zealand. Birds from one site are marked with coloured or numbered leg bands in anticipation that they will be seen and recorded at another stage in their migration. Satellite transmitters can track an individual bird over its route, but until these devices become lighter, they can only be used on large birds, such as albatrosses. Tiny geologgers may be attached to smaller species, but require the bird to be recaptured for the data to be downloaded.
Migration by birds is the regular, repeated, seasonal movement of populations from the region where they breed to another location, then back again in time to breed. In the New Zealand context this movement may be to or from the northern hemisphere (trans-equatorial migration), other parts of the southern hemisphere (for example, trans-Tasman migration) or the tropics. It may also be within a country (internal migration), including movement from higher ground to lower areas (altitudinal migration).
Migration is more common away from continents with big seasonal changes in temperature and food availability, than from temperate regions like New Zealand.
Most migrating birds move to a more favourable climate as winter approaches. However, some east–west migrants do not go through great changes in latitude, but they take advantage of abundant food supplies away from their breeding region. While there, they feed, moult and build fat reserves in preparation for the return migration and a new breeding season. In some species, young birds stay at the non-breeding site for one or more years before returning to their birth region to breed.
A New Zealand novel of the 1930s, The godwits fly by Robin Hyde, explored the life of many New Zealanders of English origin who, like godwits, were torn between two distant parts of the world.
Birds that migrate from as far away as the high Arctic to New Zealand risk the perils of long-distance flight for good food supplies. The alternative, remaining in the Arctic, would mean experiencing extensive snow cover, long hours of darkness and sparse food supplies.
New Zealand has plenty to offer migrating birds in their non-breeding season. Some inshore- and offshore-foraging seabirds that breed in the far north or far south take respite in the milder conditions around New Zealand. To waders, New Zealand’s 300 estuaries (a total area of 100,000 hectares of estuaries and coastal mudflats) provide highly productive feeding grounds for tens of thousands of migrants during the southern summer. The fact that New Zealand was free of predators such as stoats, cats and dogs until relatively recently was another advantage. Among the most important sites are the Kaipara Harbour, Manukau Harbour, the Firth of Thames, Farewell Spit, Golden Bay, the Avon–Heathcote Estuary and Lake Ellesmere.
Most of the Arctic waders travel to and from New Zealand via the western edge of the Pacific, on the East Asian–Australasian Flyway, making coastal stopovers to rest and feed along the way. The staging sites are vital for the birds to be able to continue their journeys and return home to breed again.
Pressure from expanding human populations at many sites (some of which are adjacent to huge cities) has led to a steady deterioration of the birds’ habitat. A combination of housing and industrial development, pollution, land reclamation on mudflats, changes to water flows and increased risk from predators at these stopping points has meant that fewer birds can feed and rest before the next stage of the journey. The result has been a decline in bird populations.
Several international organisations have attempted to protect some waders’ habitats along the course of flyways such as the East Asian–Australasian Flyway. By 2006, six New Zealand sites were included in the Ramsar Convention as Wetlands of International Importance: the Firth of Thames, Kopuatai Peat Dome, Whangamarino, Manawatū Estuary, Farewell Spit, and Waituna Lagoon.
Migratory waders that arrive and form large flocks at feeding sites through the southern summer include 90,000 bar-tailed godwits, 35,000 lesser knots and 5,000 ruddy turnstones. Another half-dozen wader species are regular visitors (50–700 birds). These include the Pacific golden plover, the red-necked stint, the whimbrel and the sharp-tailed sandpiper. A further dozen or more wader species are occasional visitors, along with other rarer travellers that have strayed from their usual route.
Before Polynesian voyagers set off to explore the Pacific, the sight of godwits flying south through the central Pacific gave them a clue that land lay to the south – Aotearoa (New Zealand).
At least one exceptional long-distance migrant makes the journey from the Arctic without any stopovers. The eastern bar-tailed godwit makes a record-breaking non-stop flight over the Pacific Ocean from Alaska to New Zealand. This journey of 11,000 kilometres takes eight or nine days and nights of continuous flight, averaging about 56 kilometres per hour. The departure is timed to gain assistance from favourable winds. The birds arrive exhausted and weighing less than half their original weight. They remain for around five months before making the return journey along the western edge of the Pacific.
Skuas are gull-like birds that steal other birds’ food, eggs and chicks. Skuas of both hemispheres visit New Zealand waters. The Arctic skua, the less common Pomarine skua and the long-tailed skua come south in the southern summer. The South Polar skua and subantarctic skua move north during the southern winter.
A tiny Arctic tern found at Mason Bay, Stewart Island, in December 2003 had been banded five months earlier as a chick in Hälsingland, Sweden. It must have flown down the Atlantic and across the southern Indian Ocean and Tasman Sea – an estimated 25,000 kilometres.
Several species of tern migrate to New Zealand each year in small numbers. These include the common tern and the Arctic tern, both from the Arctic region, and the little tern and the white-winged black tern from Asia. Some of these mix with flocks of New Zealand’s native white-fronted terns.
Several million seabirds breed around New Zealand, and then migrate across the equator to the far north Pacific. Others move east or west from New Zealand across the Tasman Sea, or to the eastern Pacific and western Atlantic.
In contrast to the solitary breeding habitat of most Arctic waders, many albatrosses and petrels that breed in the New Zealand region nest or burrow in dense, noisy colonies on small, predator-free islands. During this time they compete for food within reach of the island.
When breeding is over they have no need for land, and many petrel species migrate to other food-rich areas of ocean or continental shelf. The sooty shearwater or tītī, Buller’s shearwater, flesh-footed shearwater and the mottled petrel move to the northern Pacific. Birds such as the Cook’s petrel, black petrel, Chatham petrel and white-faced storm petrel migrate to the eastern and tropical Pacific. Other birds, including both royal albatross species, migrate to the Atlantic and Pacific coasts of South America, or, like the Hutton’s shearwater, to the waters around Australia.
After fledging, the Campbell albatross flies from Campbell Island to the subtropical Pacific for the winter, then joins adults off the east coast of Australia before returning to breed. Some southern-breeding petrels such as the Antarctic prion move north to New Zealand waters during winter.
Most petrels fly mainly by soaring or gliding, using the wind’s energy instead of their own as much as possible. Also, they rest and feed at sea, so their migration flights are less demanding than waders’.
Most juvenile and some adult white-fronted terns cross the Tasman Sea to winter on the Australian coast. New Zealand-hatched Australasian gannets and one wader species – the banded dotterel – also do this.
Two species of cuckoo are the only land-based New Zealand species that migrate overseas. The shining cuckoo or pīpīwharauroa winters in the western tropical Pacific – from Indonesia to the Solomon Islands and Bismarck Archipelago. The long-tailed cuckoo or koekoeā tends to winter further east, from Palau (Micronesia) across to the Marquesas and Tuamotu Islands of French Polynesia. On their return to New Zealand, their distinctive calls are a sign of spring, as in the proverb:
Ka tangi te wharauroa, ko ngā kārere ā Mahuru.
If the shining cuckoo cries, it is the messenger of spring.
For them, the breeding season is not arduous as they lay their eggs in the nests of other birds, which unwittingly rear the chicks.
As well as migrants to and from New Zealand, there are bird species that migrate within the country from one region to another. The estuaries and coastal sites that feed so many international wader visitors through the summer are used in winter by similar numbers of New Zealand waders that breed in other parts of the country. Most of these waders and other species are able to both breed and winter over in New Zealand because of the temperate climate and the diverse habitats, which offer suitable breeding environments as well as concentrated winter feeding grounds.
The native South Island pied oystercatcher is the most numerous internal migratory bird. After breeding on South Island riverbeds, farmland and coasts, the majority of the population of 112,000 assembles at North Island wintering sites. The less common variable oystercatcher, which is endemic (found only in New Zealand), tends to move more locally.
The pied stilt, thought to have colonised New Zealand around 1800, now numbers about 30,000 birds. Most migrate from breeding grounds on riverbeds of the South Island and southern North Island, to northern harbours. Others breed on the coast and are less migratory. The endemic black stilt, of which there are fewer than 200 in the wild, tends to move to sites close to its breeding grounds after it has raised its young.
Most banded dotterels that have not flown to Australia for the winter move to coastal and estuarine sites around New Zealand. Others remain on river deltas and pastureland through winter. The rare endemic New Zealand dotterel has two subspecies. The 2,200 or so northern birds mainly stay year-round near their breeding sites. The scarcer southern birds are now confined when breeding to Stewart Island, with numbers hovering around 260. They breed on summits above the bush line but move to coastal wintering sites, including the Southland coast.
The small endemic wrybilled plover, ngutuparore, breeds on braided rivers near the Southern Alps, then flies to tidal harbours in the North Island or the north of the South Island. This species is notable for being the only bird in the world with a sideways bend to its bill. It has a population of 5,000.
The endemic black-billed gull breeds on braided riverbeds and other inland sites, and it moves to coastal sites for winter.
Some kingfishers (kōtare) breed around coasts and estuaries, while others breed at inland sites. Those that breed at high altitude tend to move to lower land in winter.
Songbird species, such as starlings, blackbirds and thrushes that move long distances in the northern hemisphere abandoned migration when introduced to the more temperate New Zealand climate. Introduced ducks and geese also stopped migrating, although some move seasonally between breeding grounds and aquatic areas.
One native songbird that could be considered migratory is the silvereye or tauhou, which is a relatively recent colonist from Tasmania. After the breeding season they form flocks and move around the country. The Tasmanian population of the same sub-species migrates to Queensland.
If birds are to survive their flight and arrive at their destinations, they need to know when to go, and to be able to manage their energy needs. Preparation is crucial as journeys are long. For instance, the bar-tailed godwit flies 11,000 kilometres from Alaska to New Zealand, while the Arctic tern flies more than 17,000 kilometres.
Birds need a trigger to prepare for migration. Changing day length between the seasons provides this by setting off hormonal activity that influences birds’ physiology and behaviour. Experiments with birds held in outdoor cages showed that at the appropriate time to migrate they became agitated, slept less and started facing the direction of their migratory route. This behaviour is known as zugunruhe (from German), or migratory restlessness.
Migrating to or from New Zealand means long flights over the ocean. While seabirds may be able to feed en route, other birds depend on their ability to stockpile and conserve energy. Before departure and at stopovers, they go into a feeding frenzy, putting on weight at a great rate.
The bar-tailed godwit puts on 60%–70% of its weight in preparation for migration. By the time to leave, 55% of their body mass is fat. This enables them to fly non-stop over the Pacific Ocean from Alaska to New Zealand, whereas most other Arctic migrants follow the East Asian–Australasian Flyway along the western Pacific edge, where they can make stopovers to feed.
Fat is light and yields about eight times as much energy per gram as protein (muscle). As weight is a big cost in flight, carrying fat rather than protein or carbohydrate for energy needs is an advantage. Many birds also grow bigger flight muscles with the help of natural steroids, so they have extra power for the demanding flight. Some weight is also saved by shrinkage of organs not needed on the flight, such as the gut, liver and kidneys.
When Arctic migrants return from New Zealand to their breeding grounds, they need to have enough reserves for the start of breeding, as food supplies in the high Arctic spring are unreliable. That is why even the bar-tailed godwits take the long route north, stopping to feed at estuaries on the coast of the Yellow Sea.
Not all birds have equal access to food during stopovers. There is great competition when there is not enough to go around, with larger or more dominant birds making it difficult for inexperienced or weaker birds to get their fill. The stakes are high, as birds that run out of energy during a crossing will drop out of the sky.
Birds have developed a variety of ways to reduce the amount of energy they need for flight. Some species fly in formation, which reduces expended energy by around 20%. Non-gliders can increase their speed by flying at altitudes of around 1,500 metres, in air that is less dense than at sea level. Many seabirds glide and use wind and wave pressure for ‘dynamic soaring’. This needs much less energy than flapping their wings. Soaring on rising thermals is possible for birds crossing hot continents, and many birds also make good use of tail winds to reduce the amount of energy and time.
How do migratory birds know where to go and how to get there? Is it learned or innate? In some species, young birds make the journey alone, weeks after the departure of their parents. In those cases at least, some form of genetic imprinting must be involved, but external navigational aids are still needed.
Some species migrate in large flocks, with experienced birds showing the way.
In other species, when birds migrate for the first time without a guide, a type of clock-and-compass orientation is used. This seems to be genetically programmed, with an instruction equivalent to: ‘Fly south-east for three days, then fly due south for one day.’ The birds may then build up an internal map as they go, using landmarks, which they remember for next time. Co-ordinates (equivalent to latitude and longitude) could be established once both ends of the route are known, and these would enable a bird to get back on course even if blown beyond their known landmarks. In reality, latitude can be determined more readily than longitude.
Migratory behaviour and navigational ability have evolved differently across groups of birds. Also, some navigation strategies work better in one part of the world than another. For example magnetic cues may be weaker or more confused, and constant cloud cover or fog can cause difficulties for techniques that depend on the sun or stars.
Experiments show that many migratory birds can detect magnetism, but how they do this is not certain. One possibility is that it involves the cells with iron-rich magnetite crystals found in the brains of many birds and other animals (including humans). Another interesting idea has come from a study of silvereyes. It suggests that they see the earth’s magnetic field as a faint colour gradient, which indicates the field’s strength and direction.
While we think of magnetism as being useful for giving a compass direction, some species also use the variation in the strength and inclination (angle of dip) of the magnetic field to build parts of their map.
The earth’s magnetic field is not altogether constant or reliable, and its information changes through time. The position of the magnetic poles in relation to the geographic poles changes year by year, so a bird may need to adjust its bearings within its lifetime. The strength of the field changes over centuries, and in the last 150 years has become weaker.
In addition, every so often (about 10 times over the last three million years) the magnetic poles reverse. This is accompanied by a great disruption to the magnetic field, which lasts for hundreds of years. So any inherited information about using magnetism needs to have inbuilt flexibility. Birds apparently achieve this by comparing magnetic field information with cues from the sky.
As the earth spins, the night sky appears to revolve (so-called celestial rotation). Stars closest to the South or North poles move the least, while stars closest to the equator move fastest and furthest. Young birds build up a picture of the position of the centre of rotation (over the nearest pole), which gives them the north–south axis. To use these celestial cues, young birds need to view the rotating sky prior to migration, for it to become imprinted.
During the day, the sun’s movement across the sky gives orientation signals that vary with time of day. Like many animals, birds have internal clocks that help interpret these signals. For example, they know that in the late afternoon the sun’s position is more to the west than it is at midday or in the morning. The height of the sun at midday indicates latitude if the time of year is known. However, knowing the time becomes complicated during the course of migration. Local time may change if the bird moves longitudinally on the east–west axis, and day length changes as birds change latitude.
Birds need a good memory to retain details about their route, and migratory birds do better on memory tests than non-migratory ones.
Migratory birds gain information from the sun better than humans can. They can detect the changing pattern of polarised light across the sky throughout the day, including the period before sunrise and after sunset. Birds need only a small area of blue sky to see light changes, so this is particularly useful when the sun is obscured by patchy cloud cover.
Magnetism and celestial rotation each have limitations, but used together they become more reliable. Birds constantly check one system against the other and adjust their direction. They may also rely on one system more than another depending on where they are.
Several sources of information can help birds build a map as they fly. These include:
Berthold, P., and others, eds. Avian migration. Berlin: Springer, 2003.
Ell, Gordon. Seashore birds of New Zealand. Auckland: Bush Press, 1984.
Heather, Barrie D., and Hugh A. Robertson. The field guide to the birds of New Zealand. Rev. ed. Auckland: Viking, 2000.
Higgins, P. J., and others, eds. Handbook of Australian, New Zealand and Antarctic birds. 7 vols. Melbourne: Royal Australasian Ornithologists Union and Oxford University Press, 1990–2006.
Power, Elaine. Sea and shore birds of New Zealand. Auckland: David Bateman, 1990.
Robertson, Hugh A., ed. ‘Wader studies in New Zealand.’ Special issue. Notornis 46, part 1 (1999).