Are Some Birds Not Flying South Due to Warming?

James Owen
for National Geographic News

November 22, 2005

European songbirds are canceling their annual winter breaks in Africa,
preferring instead to fly to Great Britain, bird experts say.

The surprising detour in European warbler migrations was revealed by data from an ongoing survey that involves bird-watchers across Britain.

The survey shows that rising numbers of warblers, which spend their summers in northern Europe, are no longer flying south to Africa and the Mediterranean each fall.

"I am amazed by the numbers of warblers that were reported," said Greg Conway, a researcher with the British Trust for Ornithology (BTO), who runs the survey.

It's as if the birds are now saying, "Let's not bother to go all the way to Africa this winter," he said.

Eleven species of migratory warblers wintered in Britain last year. Reports from more than a thousand British birdwatchers included sightings of around 1,500 blackcaps (Sylvia atricapilla) and almost 1,000 chiffchaffs (Phylloscopus collybita).

"The blackcaps are almost exclusively birds which breed in Germany and central Europe and are coming [to Britain] for the wintertime instead of going to Africa," said BTO songbird expert Graham Appleton.

Appleton says the wintering chiffchaff population is made up mainly of British birds that are staying put, plus some visitors from mainland Europe.

Other reported species usually not seen during the British winter included willow warblers, garden warblers, reed warblers, and whitethroats.

Winter Warblers

The warblers' migration to Britain instead of Africa could be a response to global warming.

Increasingly mild winters mean the birds can now cope with Europe's coldest months, giving them a head start the following breeding season.

"The decision not to travel south across the Sahara Desert may have some clear benefits: boosting overwinter survival and leading to an increase in breeding populations," Appleton said.

Bird experts have found that some warblers have a genetic disposition to fly west instead of south in the fall. Previously these birds would have perished in the cold European winter. But many now survive and pass this migratory trait to their offspring.

Appleton says the birds that stay in Europe are better placed to judge when conditions are right to return to their breeding range in spring.

"If you migrate down to Africa south of the Sahara, you have no idea what the weather is doing in Germany or surrounding countries," he said.

Warblers wintering in Britain can claim the best breeding sites, he said. "Because they have the best breeding sites, they have the best productivity.

"And, because it's a genetic trait, they are pumping out more and more kids which come [to Britain in the winter]."

As for food, the BTO says bird feeders and berry-laden shrubs in gardens are helping blackcaps get through the lean winter months.

Sewage-treatment plants are becoming a popular winter haunt for insect-eating warblers such as chiffchaffs. Warm water pumped into filter beds (layers of sand or gravel for filtering water) at these treatment plants promotes aquatic insects in winter. "Insects swarm from the filter beds into foliage close by," Appleton said.

Warblers aren't the only British birds known for making the most of milder winters, he added.

"Back in the 1960s a large percentage of our song thrushes flew to Spain and Portugal for the winter. But it's a much, much smaller percentage now."

Pros and Cons

Climate change, however, can also cause problems for migratory birds, because it brings unpredictability, experts warn.

"If you're going to migrate, then not knowing what the weather's going to do and not knowing what the weather's going to be like at your stopping-off points is really bad news," Appleton said.

Last month the U.K government's Department for Environment, Food and Rural Affairs published a report titled "Climate Change and Migratory Species."

The report says barriers to bird migration may become more severe because of climate change.

For instance, reduced rainfall and increased desertification in Africa's Sahel region—an important "refueling" area for Europe-bound birds before crossing the Sahara—could have severe impacts.

"Breeding numbers of species such as whitethroat are substantially lower in drier years, so further declines in trans-Saharan migrants might be expected with climate change," the authors wrote.

Long-distance migrants are also vulnerable to climate change in their summer breeding range.

For example, the arrival of European swallows from Africa is timed to coincide with the emergence of insects in spring.

Biological records show that insect species are appearing six days earlier on average for each degree Celsius rise in temperature. But swallows' egg-laying dates have hardly changed.

This, the authors wrote, could "lead to a mismatch between the birds and their prey."

Meanwhile birds like blackcaps and chiffchaffs, which are wintering closer to home, are less likely to face bad timing with their food supply.

Experts say shorter trips could be the way forward for migratory birds in a warmer world.

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