Miranda's migratory birds

The Firth of Thames, with its 8,500 ha of wide inter-tidal flats, attracts thousands of migratory wading birds.

Some make the arduous 10-12,000 km journey south from the Arctic circle to arrive in the spring and fly north again in the autumn; others fly 1,000 km north from the braided river systems of the South Island in the autumn and return in the spring.

In October it's a changing of the guard as the arctic migrants like the eastern bar-tailed godwit, the red knot, the turnstone and the red necked stint arrive and the birds from the South Island like the wrybill, South Island pied oystercatcher and the kōtuku which have over-wintered in the Firth, fly back to their southern breeding grounds.

A welcome ceremony, organised by the Pūkorokoro Miranda Naturalists Trust, is held in October. In March, another ceremony marks the autumn change-over as the godwits, the turnstones, the stints, and others head off and the birds from the south move in.

Favoured destination

The shell banks which have formed along the coast provide safe roosting for birds at high tide and make for easy bird viewing. The shell banks are an internationally significant landform as they are probably the best example anywhere of a Holocene coastal strand accreted by a combination of gravel and shell beach ridges. These are known as Chenier Plains. Within New Zealand Chenier Plains are reported as a Naturally Uncommon Ecosystem which is endangered, by the action of drainage, mangrove invasion and stock damage.

The Firth of Thames intertidal area is listed as an internationally important wetland under the criteria of the Ramsar convention on Wetlands of International Importance. The tidal mudflats of the Manukau Harbour are another favoured destination for migratory birds.

Hazardous journeys

The tidal flats and mangroves between Kaiaua and Thames support up to 40,000 birds during all or part of their annual cycle. Sixty different species have been recorded, 43 of them wading birds. Birds New Zealand (also known as The Ornithological Society of New Zealand) conducts a census in November and June. These counts which have been ongoing nationally in one form or another since 1983 are an important indicator to tell us which species are where how many of them are there and how many are at the Firth each year.

It is not just wind, weather, and the huge distances they have to contend with. Large numbers of arctic migrants are shot for food each year as they travel via the East Asia-Australasian flyway. Equally important is the ongoing impacts of the loss of their vital tidal mudflat stopovers which are being reclaimed for aquaculture, airports, or other building developments.

The impacts of climate change on migrating birds are multiple. Impacts such as changing sea levels can affect access to food resources on stop overs and wintering sites. Where reclamations have removed the upper zones on beaches access to high tide roosts can be reduced. In the Arctic breeding grounds the changes in the pattern of when invertebrates (a key found sources for developing chicks, and their parents) emerge could lead to starvation events or failure for growing chicks to develop fully before leaving on migration. Many of these effects are not consistent across the arctic and so could have different levels of effect on different species.

Banding birds for answers

The godwits are the most common arctic wader to arrive in August and September each year. They arrive in New Zealand after making one spectacular nonstop journey from Alaska. To make this journey successfully they double their normal bodyweight to be able to complete this 11 000 km flight. When bar-tailed godwits leave NZ in Autumn each year that have bulked up so they can fly to the Yellow Sea region in China in one flight and then onto Alaska.

The ability of birds to migrate from one part of the world to another has always fascinated people. Why and how do they do it and which routes do they take? Increasingly sophisticated bands and GPS tags are being used to unravel the details of migrations of different species that move up and down the East Asian-Australasian Flyway. Banding is carried out over the summer by volunteers with the New Zealand Wader Study Group to find out just where the birds from Pūkorokoro and elsewhere go on their way to and from the arctic circle. Nets are used to capture birds as they rest at Pūkorokoro’s high tide roosts and a small white plastic flag is attached with a band to the upper leg. This lets people in other places know these birds have come from New Zealand.

As reports of sightings come in, a more complete picture of their perilous journeys can be put together, along with statistics to help persuade foreign governments to preserve essential mid-journey habitats. Additionally tiny transmitters are put on some of the larger waders like the curlews. Such detailed data allows for detailed examination of how birds use space, their speed and height at which they fly.