Early Birds: Is Warming Changing U.K. Breeding Season?

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"Usually it's increasing day length that triggers egg laying, but if you get exceptional spells of warmth, along with an unseasonal superabundance of food, this can override day length as the main factor."

This is also the conclusion drawn from data provided through the BTO's Nest Record Scheme. The largest scheme of its type in the world, it holds over 1.2 million individual nest histories for British birds since 1939. Some 30,000 new records are provided by volunteers each year.

"The Nest Record Scheme has provided one of the clearest indications to date of the impact of climate change on British wildlife," said Humphrey Crick, the BTO's senior ecologist. "About a third of the 60-odd species we've looked at show statistically significant trends towards earlier egg laying. This is strong information because it comes from all over the country and isn't restricted to just one study site."

Crick says that birds are laying between one and three weeks earlier than they did 15 years ago. For species like the barn owl, which can produce a second brood, this will almost certainly prove advantageous.

"For barn owls, the lack of severe winters plus earlier springs are obviously a good thing," said Crick. "It will allow them to fit in more second broods than they normally do."

But Crick says for some birds milder conditions aren't so helpful.

Great Tit

He said: "A single-brood bird like the great tit only has one shot at breeding as they depend on a big splurge of a particular type of caterpillar. Although both the eggs and the caterpillars are appearing a week or so earlier, the trouble is that in favorable conditions caterpillars are able to halve their developmental time from 50 to 25 days. Meanwhile great tits are stuck at about an 18-day incubation period. So if the chicks are a bit late they can hatch after the caterpillars have already disappeared."

Changing weather patterns are also influencing Britain's bird life in other ways, for instance by turning rare migrants into breeding residents. Since the 1990s both little egrets and Mediterranean gulls have established colonies after arriving from southern Europe.

And last year a pair of European bee-eaters caused a stir when breeding successfully in England for the first time in almost 50 years. The birds, which are hard to miss with their kaleidoscopic plumage, aren't normally found farther north than southern France.

"There are other species waiting in the wings to come across from mainland Europe," said Crick. "They include the black woodpecker and fan-tailed warbler which have reached the English Channel. It's a very interesting time to be a bird-watcher."

On the other hand, species like the dotterel and snow bunting look set to head northwards, leaving British shores for colder climes.

It may be an interesting time to be a bird-watcher in Britain, but they will have to think about investing in a new bird book. Current editions may soon be out of date.

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Additional Information from Related Web Sites:
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National Audubon Society
Environmental Protection Agency: Bird Conservation

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