Are Some Birds Not Flying South Due to Warming?

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"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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