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.
Reasons for migrating
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.