Nights are growing cooler, days shorter. Nature's signals are telling the wading birds that fly from the Arctic tundra to spend summer in New Zealand that it's nearly time to go.
Admiring the aerial ballet on the Firth of Thames, a shorebird sanctuary of global significance, Keith Woodley wonders how many of the thousands of Arctic migrants enjoying this golden summer will return.
To prepare for their 16,000km journey north, they've been fattening up on pipis, worms and other rich pickings in our northern harbours and mudflats: godwits, red knots, colourful turnstones and Pacific golden plovers down to the tiny red-necked stint.
Godwits are famed for their ability to fly here direct from their breeding grounds in Alaska. But on the way home, like the other birds, they must stop to refuel if they are to reach the Arctic Sea in suitable condition to breed successfully.
Their key "gas station" is 10,000km away in the Yellow Sea, where rampant development of the coastline and tidal mudflats by China and Korea is threatening the future of many migratory bird species.
Among them are our most populous Arctic visitors, the red knots and godwits.
Red knot numbers in New Zealand have declined from about 58,000 to 30,000 in 20 years.
The International Union for the Conservation of Nature (IUCN) is warning that the red knot population faces collapse unless remaining intertidal areas are safeguarded at its main stopover in Bohai Bay, on China's eastern seaboard.
An estimated 60 per cent of New Zealand red knots converge on Bohai Bay on their way to breeding grounds on the Chukotka Peninsula, the eastern tip of Siberia.
Bar-tailed godwit numbers are holding up locally but declining globally and expected to soon hit a downward spiral here. Eighty per cent of godwits stop over at the Yalu Jiang estuary, on the border between China and North Korea.
Woodley, the manager of the Miranda Shorebird Centre, reveals their plight in his latest book, Shorebirds of New Zealand, an engaging and beautifully illustrated account of the lifestyles and quirks of our shorebirds - and the threats to their survival. In this country, pressures of pollution and coastal development are common enough (planned housing developments on the Manukau Golf Course and at Te Arai Pt near Mangawhai are current flashpoints) and the outlook for many shorebirds is distressing - the New Zealand fairy tern is on the cusp of extinction; wrybills and dotterel numbers are a worry. But the pressures here pale in comparison to the rate of habitat loss in the Yellow Sea - a threat to "our" birds we have little power to reverse.
The pace and extent of reclamation around the Yellow Sea is astonishing. An average of 285sq km in coastal China is being reclaimed each year, with 57 per cent of wetlands already lost, a report cited in China Daily claims. Habitats are disappearing at such a rate that researchers fear population collapse is imminent for many species. The IUCN says at least 24 species of shorebirds in the East Asian-Australasian Flyway [the route followed by Australasian migratory birds] are heading towards extinction and many others face "exceptionally rapid" losses of 5 per cent to 9 per cent a year. The red knot is declining at 8 per cent to 9 per cent a year; the godwit at around 5 per cent.
Woodley has witnessed the decline of Yalu Jiang, where our godwits stop, in several visits. The nature reserve where they forage and roost at the mouth of the Yalu River is being squeezed out by coastal development and reclamation for the Donggang development zone, he told the Weekend Herald. Massive seawalls for fish farms are being constructed in an area that traditionally holds the greatest concentration of godwits.
The reserve boundaries were last year reduced by 20,000ha and reclamation has narrowed the estuary at its widest point from 3.5km to 2.5km. Fish farms now run the entire 60km length of the reserve, forcing the birds to find alternative roosting sites inland or endure regular disturbance. But 30,000 people work inside the reserve. "Sometimes there is nowhere for the birds and they have been observed circling overhead for up to an hour until the receding tide relinquishes some living space," Woodley writes.
"Given the reason the birds are there - to store energy reserves prior to arrival on the breeding grounds - this waste of energy may have serious consequences later in their annual cycle. Birds are also losing foraging habitat, a lot of it and quickly."
But Woodley says the threat to the godwits at Yalu Jiang is only "mildly disturbing" compared to the scale of development in Bohai Bay. To cope with population growth and land development pressure, the Chinese Government in a succession of five-year plans has encouraged reclamation for urban and industrial use and vast container shipping ports, as well as seawalls for marine farms. For developers, dredging or sucking silt and sand from the seabed can be half the cost of sourcing land-based fill.
Relocation of heavy industries including oil refineries and steel and car plants to coastal areas has also made the Bohai Sea one of the most polluted in the world. There is no end in sight, with China's coastal population expected to rise from 550 million to 700 million by 2020.
Since 1994, two enormous reclamation projects on opposite sides of Bohai Bay - Tianjin Binhai and the Caofeidian New Area - have taken 649sq km of mudflats (nearly twice the size of the Manukau Harbour), China Daily reported.
Red knots and other migratory species are being squeezed into a 20km by 3 km strip of mudflats between the two projects and more reclamation is scheduled, with obvious implications for food supply and conditioning.
The rogersi sub-species of red knot, which comes here, was last year declared a New Zealand native, because it spends at least 50 per cent of its time here and because the New Zealand flocks make up more than 25 per cent of the entire rogersi population. But at the same time it was added to our threatened birds list - classified as nationally vulnerable - because of the impact of the loss of stopover sites.
The red knots' problem is their highly specialised diet, Woodley explains in Shorebirds. With their relatively short beaks, they survive outside their breeding grounds on shallow-buried, thin-shelled bivalves (such as pipis) and small crustaceans "that are swallowed whole then crushed in the bird's large and powerful gizzard". Their picky diet means there are few habitats where they can congregate in large numbers. "The annual cycle of knots is a story of living on the edge and being totally dependent on a handful of suitable locations."
For evidence of the effect on bird populations when stopover habitats are diminished, Woodley points across the Yellow Sea to Saemangeum on the South Korean peninsula, once the largest and most important staging site in the Yellow Sea. A 33km-long seawall across the mouth of two rivers, completed in 2006, and reclamation within is destroying 41,000ha of wetlands with a devastating impact on wildlife - including the red knots' larger cousin, the great knot, whose global population has fallen by 22 per cent. The decline highlights that the birds can't simply go elsewhere, he says. For another species, the spoonbilled sandpiper, the loss of the Saemangeum estuary has worsened an already serious plight and it is on the verge of extinction.
But Woodley says species can recover if protections are put in place in time.
At its conservation congress (held every four years) last September, the IUCN called on governments in the Yellow Sea to ensure better conservation of remaining intertidal habitats. Woodley is heartened that both China and South Korea supported IUCN resolutions which asked their governments to halt further mudflat reclamation at key sites until they are ecologically assessed. Governments were also called on to have at least 10 per cent of the remaining intertidal zone in protected management by 2020.
But Woodley says international pressure will be needed to ensure both countries follow through. South Korea, for one, has a reputation as a laggard, having failed to act on undertakings it made to protect Saemangeum. He wants the Government to exert diplomatic pressure, exploiting our close relationship with China in particular. "I would love to see these issues raised with the Chinese Government, to get them flagged at the top level. Time is running out for some of our birds."
• Shorebirds Of New Zealand - Sharing The Margins, by Keith Woodley (Penguin $50).
• The annual Farewell to the Birds is at the Miranda Shorebird Centre on Sunday, March 3 at 10am. Guest speaker is Massey University PhD student Jimmy Choi, who is researching godwit foraging habitat at Yalu Jiang.By Geoff Cumming Email Geoff