MOUNT MORELAND, South Africa — Wetlands — critical for the health of South Africa's coasts and river systems — already have been degraded or seriously altered by human activity, and experts fear global warming threatens them further.

As talks to shore up the international response to global warming entered their second and crucial week in the South African coastal city of Durban, environmentalists led a tour of a wetlands area nearby.

It's a spot where spectators start coming an hour before sunset. They set up deck chairs or spread blankets, take a bottle of white wine from the cooler and a block of cheese or snacks, settle down with binoculars, and hope.

This Sunday, the barn swallows didn't put on their show.

Millions of birds, having migrated more than 5,000 miles from Europe and Britain for the southern summer, usually roost in the tall reeds poking through the surface of Lake Victoria at the foot of a hillock called Mount Moreland.

The weather is chilly — at least 9 degrees Fahrenheit (5 degrees Celsius) cooler than normal, says resident Angie Wilken. And the migration is two or three weeks late this year, she says. The birds are still leaving Europe.

"I'm constantly questioning. Is it just the weather? Is that really it?" she says.

Stormy future
Scientists are reluctant to blame climate change for any single unusual weather event or short-term departure from the norm. But studies and computer modeling show that man-made emissions of carbon dioxide are disrupting normal weather — both hot and cold spells — around the globe, causing more storms, droughts and floods and affecting wildlife.

If the planet continues to warm at the current pace, one-third of all animal and plant species may become extinct by the end of the century, according to an authoritative panel of U.N. scientists.

In Durban, climate ministers and other top officials, including U.N. Secretary-General Ban Ki-moon, are expected for the last week of negotiations.

They are under pressure to conclude by Friday with pledges to cut greenhouse gas emissions after their current commitments expire next year under the 1997 Kyoto Protocol.

They also need to make progress on raising billions of dollars to help poor countries cope with global warming if the talks are to be deemed a success.

On Mount Moreland, the roosting of barn swallows, which fill the sky and then swoop onto the sage-green reeds in a single massive ball up to 3 million birds thick, is one of nature's most spectacular displays.

"The horizon just starts spewing them over the top. Then they form a tighter unit, moving left and right. And as they turn, they peel and drop into the reeds as fast as stones dropping. And if you're not watching you miss it," says Wilken, who watches it nearly every evening from October through April.

'Bedtime stories'
The sound before the birds settle for the night is like water running, she says. "We call it bedtime stories."

The vegetation of wetlands like Lake Victoria provides a haven for birds and wildlife, purifies water of nutrients spilled from agriculture and provides a livelihood to poor people who plant its fringes with vegetables or marigolds.

Experts worry about predictions that as the Earth's average temperatures rise, South Africa's east coast will become more arid and the west coast around Durban will get more rainfall, raising the risk of floods and erosion.

"If the wetlands dry out, the impact will be huge on small farmers who exist close to the line," says Damian Waters, a wetlands expert for the Wildlife and Environment Society of South Africa.

Waters' group and others are working to protect the wetlands and the 250 estuaries that break up South Africa's coastline. Collaborating with local and national authorities, the nonprofit groups are producing detailed topographical maps of wetland areas and how they integrate with farmland and industry.

They encourage big water users in the area to conserve water and replant climate-resilient indigenous vegetation, which has struggled to compete with invasive foreign trees and shrubs that use more water.

Disappearing wetlands could mean trouble for the barn swallows of Mount Moreland, where Wilken won approval from landowners over the years to clear an area for bird watchers to view the natural wonder of the roost.

Some 40 million European barn swallows pass through the area each year, she says, maintaining a low population of summer insects. "They do a huge service when they come to this country," she said.

But she's concerned about a new commercial development and a nearby airport, whose approach path is directly over the field of reeds and whose runways disrupt the normal flow of rainwater into the valley.

"I'm constant worrying," she says. "Is nature going to prevail?"