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

James Owen in England
for National Geographic News
June 3, 2003

For a bird, the bleak midwinter wouldn't seem an ideal time to go about the precarious business of starting a family. But recent winters haven't been particularly bleak in Britain, and last Christmas scores of fluffy owlets grew fat on mice, voles, and baby rats caught by parents that spotted a breeding opportunity too good to ignore.

Scientists at the British Trust for Ornithology (BTO) say 2002's mild temperatures and an explosion in rodent numbers sparked an unseasonal baby boom in the country's barn and tawny owl populations.

"We had five cases of tawny owls laying eggs from Christmas week right through to the end of January," said David Glue, a research biologist with the bird conservation charity. "This is exceptional—usually they don't start nesting until the second or third week in March."

Glue says the latest in a series of warm years—2002 was the fourth hottest in Britain since records began in 1659, with five of the six warmest occurring since 1990—contributed to a huge increase in the rodents that owls prey on.

"An early spring and a late autumn produced a heavy crop of beech mast, hazelnuts, conifer seeds, and other natural fruits, so we had large numbers of voles, mice, and young brown rats," said Glue.

Biologists estimate that two billion field voles were born in Britain last year. The usual figure is around 700 million.

The barn owl—a ghostly presence at dusk when it hunts on pale, silent wings—also benefited.

Glue added: "Many barn owls laid two weeks early during the mild spring which meant 10 percent of them went on to have seconds broods, some as late as December."

They included a pair in Cambridgeshire in eastern England which produced a second brood of nine owlets. Fledging successfully just before Christmas, it's the largest second brood ever recorded in British barn owls.

Winter Nesting

Until recently, winter nesting in British birds has been very rare beyond a handful of species that include the wood pigeon, feral pigeon, and collared dove. There are also cases of certain songbirds kicking off their breeding season before the arrival of spring.

Yet ornithologists believe such behavior is becoming an increasingly common phenomenon. Glue said: "We can expect earlier and earlier egg laying and cases of birds nesting right through the winter. This is a reflection of climate change through global warming.

Continued on Next Page >>


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