Follow the birds: Unravelling the mysteries of shorebirds' long-distance migrations

Several thousand bar-tailed godwits resting at high tide on Sindo Island, North Korea, on their way north to their Arctic breeding grounds. Photo / Keith Woodley

Each year, at the end of the northern summer, millions of birds leave their Arctic breeding grounds to fly south to warmer regions. Several species, mainly waders (shorebirds), migrate along what is known as the East Asian-Australasian Flyway to Australia and New Zealand, where they spend the southern summer in the region's estuaries and shallow bays. In New Zealand, the most numerous of these species are the kuaka or bar-tailed godwit (currently around 78,000 individuals) and huahou or lesser knot (about 32,000 birds).

Nearly all bar-tailed godwits reaching New Zealand fly non-stop across the Pacific Ocean from their breeding grounds in Alaska, a journey of just under 12,000km. Smaller species, such as the knot, which breed primarily in eastern Siberia, migrate down through east Asia and Australia, stopping over only briefly to rest and feed in coastal wetlands along this 16,000km route.

The birds' return journeys are equally remarkable. All species migrate north along the flyway to gather in the vast estuaries and shallow bays of the Yellow Sea, between China and the Korean Peninsula. The godwits from New Zealand do this largely in one uninterrupted flight, covering over 10,000km in 7–8 days, during which time they lose around 40 per cent of their initial body mass. This 5–7 week stopover is vital, enabling them to replenish their energy reserves before continuing on to their Arctic breeding grounds, up to 6000km further north.

But things are changing. On the birds' breeding grounds, global warming is changing the timing of snow-melt, affecting the emergence and abundance of insects on which the birds depend. They also face challenges elsewhere, not least from massive land reclamation and industrial expansion at their critical stop-over sites around the Yellow Sea, developments that are causing disturbance, creating pollution and changing tidal flows. Birds displaced by these developments cannot easily re-establish themselves elsewhere, as it simply increases the pressure on local food sources because such areas already support other shorebirds.

For many years, international shorebird specialists, including from New Zealand, have been studying shorebird migration in both China and South Korea. But whereas much is now known about the birds' stopover sites in these countries, nothing was known until recently about the situation in North Korea, a difficult place to visit.

In this month's Nature Talks, Keith Woodley, manager of the Pukorokoro Miranda Shorebird Centre on the Firth of Thames and author of Godwits: Long-Haul Champions, will talk about unravelling the mysteries of these birds' migrations. He will focus what he and his colleagues from the Pukorokoro Miranda Naturalist Trust found in five years of surveys along North Korea's Yellow Sea coast: how they got there; how what they found filled a major gap in our knowledge of this critical link in the birds' migration cycle; and how it can help conserve this remarkable system and its associated species.

His talk, Follow the Birds: Unravelling the Mysteries of Shorebirds' Long-distance Migrations, will be given in the Davis Lecture Theatre, Whanganui Regional Museum, on Tuesday, February 16, starting at 7.30pm. Entrance is free, although a koha is always welcome from those who can afford it.

Nature Talks is a series of bi-monthly talks offered by three local environmental groups — Birds New Zealand (Whanganui Region), the Wanganui Museum Botanical Group, and the Whanganui branch of Forest & Bird — working with the Whanganui Regional Museum, on topics related to New Zealand's environment and natural history, and their conservation. The talks are held on the third Tuesday of the month.