The Listener, September 18, 2010, #3671, p30
Impossible. Beyond the limits of endurance for any animal. Such were the views, just a few years ago, expressed by some biologists over the suggestion that bar-tailed godwits could reach New Zealand from Alaska in a single non-stop flight. Yet right now, somewhere over the Pacific birds are heading this way, doing just that.
A flock may consist of just a handful of birds, or a hundred or more: puffs of feathers powered by tiny cardiovascular systems burning brown fat. They will have spent the past few weeks foraging on the mudflats of Alaska’s Yukon-Kuskokwim Delta (YKD), storing sufficient layers of fat to double their body mass. Just before departure, body organs not essential for the flight will have begun to shrink. The birds’ blood system will have adapted to carry lipids more efficiently to flight muscles. Then, once wind patterns were suitable, they will have launched their endeavour – to span the Pacific in one flight.
Among the non-sceptics was American biologist Robert E Gill who, since 2006, has led a project fitting satellite tags to godwits. Nine birds tagged in Alaska and New Zealand were tracked during migration. On August 30, 2007, one female known as E7 – from her coded leg-flag – took off from Cape Avinof on the south coast of the YKD and flew into the record books. She was tracked to the mouth of the Piako River in the Firth of Thames, where she landed on September 7, having flown 11,680km in just over eight days. “Maintaining an estimated metabolic rate of 8-10 times basal metabolic rate for more than nine days represents a combination of metabolic intensity and duration that is unprecedented in the current literature on animal energetics,” says Gill.
Consider what a bird roughly equivalent to a magpie in size must cope with to undertake such a journey. It must sustain this high-energy exercise regime for over eight days; some Tour de France cyclists raise their metabolic rate by up to five times, but at the end of each day they stop exercising. The migrating godwit’s body organs undergo structural change as the flight proceeds, using protein released by tissue breakdown as part of its fuel system. It needs to remain hydrated as well as go without sleep until it lands again. And if all that was not enough, the departure mass of E7 was probably about 600g, on top of which she carried a 27g implanted tracking device.
Birds arrive here exhausted, emaciated, their hard-used wings drooping. Why do they do it? Shorebird migration is about tracking resources over different seasons, and for much of a godwit’s annual cycle that means intertidal mudflats and the seafloor dwelling fauna found there. The arctic tundra in summer provides abundant invertebrate food for breeding birds and their young. By late summer, as those resources diminish, both adults and fledglings have an exit strategy – moving to coastal mudflats to fuel up before heading south. Long before the northern winter arrives, godwits – including some just a few months old – are distributed over mudflats in New Zealand.
Just how and why this transpacific flight evolved is not clear. One hypothesis is that as long-distance migration is such a demanding business, any factors reducing risks of mortality should be favoured. Suggested advantages of a single flight include minimising time and energy costs, and avoiding predators. It is also suggested birds may suppress their immune systems during migration to save energy, and the oceanic route may provide a corridor free of pathogens and parasites.
Yet on northward migration, godwits stop to refuel along the Yellow Sea coasts of China and Korea. In March 2007, E7 took seven days to fly 10,215km from Piako River to Yalu Jiang Nature Reserve near the Chinese-North Korean border. Staging sites here are within a few thousand kilometres of breeding grounds, which means birds can arrive at their final destination with reserves of stored fat. This is important if the weather is poor and food is scarce on arrival, and also helps boost energy for breeding. For a male godwit this gives it the capacity for flamboyant display flights above the tundra, all the while sounding like a car alarm.
To sustain their incredible migrations, godwits need a set of tools and strategies. We know they have a good navigation system – a set of compasses tuned to magnetic fields as well as the sun and stars. But above all they need a good sense of weather.
Wind is an essential factor for most migratory birds – like cyclists, they prefer to avoid headwinds. Godwits departing Alaska are known to make use of a relatively predictable weather pattern each year over the North Pacific. From late August, a series of storms from the Aleutian low-pressure system passes over bird staging sites. Once a system is stationed to the east, birds catch a lift on its northerly flows. But once they head south they must pass through several broad weather zones and cannot always be guaranteed tailwinds.
Two birds that were being tracked heading southwards were seen to turn abruptly just north of Fiji and head west to New Caledonia. Weather data revealed both had encountered periods of headwinds earlier in their journey, and were then confronted with a south-easterly flow near Fiji. It suggests, says Gill, that “birds engaging in extreme endurance flights may be acutely aware of how much fuel they carry at any given time, especially near the end of such flights”.
He has noted a pattern to godwit records throughout the Pacific. Birds are rarely seen in Hawaii or in the Central Pacific, which lie within the early stages of a godwit’s journey. Sightings increase further south towards Fiji and the Solomon Islands, where birds are more likely to drop out later in the flight.
Godwits arrive in New Zealand from early September and immediately begin preparations for their departure in March. Over the next 100 days a full wing moult will replace flight feathers that have carried them over 30,000km just on migration alone. They will then moult into breeding plumage and, continuing their pattern of obesity and marathons, begin storing fat reserves. The Alaskan breeding population of godwits, source of most New Zealand birds, may number less than 100,000 and appears to be declining. Reasons for this are unclear, but it seems likely habitat degradation and loss are major factors, especially around the Yellow Sea region.
We know as many as 80% of northbound godwits stop in the region of Yalu Jiang, which, despite its reserve status, faces intense pressure from development.
Godwits are one of our cultural totems. For Maori they were birds of mystery: “Who has seen the nest of the godwit?” They are said to accompany the wairua of the departed back to Hawaiki. Robin Hyde’s The Godwits Fly, Godwit Press and the logo of NAC – the precursor to Air New Zealand – are further examples. Christchurch rings the bells when godwits arrive, yet they touch down on harbours and estuaries from the Far North to Southland, so why not celebrate them nationally? Manukau and Kaipara harbours and the Firth of Thames support over half the godwits in New Zealand; Auckland – one of the major Pacific cities – can lay claim to being “godwit central”. Why not make more of this – make it a spring festival? What better symbol than a bird that flies the length of the Pacific in one flight to get here?
Keith Woodley is manager of the Miranda Shorebird Centre on the Firth of Thames.