Bird population weathers global warming

Researchers have found that at least one bird population in England has managed to adjust to global warming.

The members of the great tit species have timed their breeding season over the last five decades so that their chicks hatch when their main food source, the winter moth caterpillar, is most abundant, the researchers reported Friday in the journal Science.

“It’s kind of good news to know that some birds can adjust,” said Anne Charmantier, an evolutionary biologist at France’s National Center for Scientific Research and a coauthor of the study.

Few wild animal populations have been monitored as long as the great tits of Wytham Woods, near Oxford. Since 1947, scientists have tracked the fate of nearly every bird, using tiny leg bracelets.

The current study relies on data going back to 1961, when the tracking methods were standardized. The researchers found that the birds modify the timing of egg laying in response to the temperature.

The birds have coped with a 4-degree rise in spring temperatures over the last 47 years by laying their eggs an average of 14 days earlier.

The population has doubled over that period to about 400 stable pairs, showing that the birds can thrive under changing conditions.

The birds seemed to be responding to early-season temperature cues that also control the caterpillar peak. The same bird that laid eggs earlier than usual in a warm year would delay breeding in a cold year.

Still, flexibility seems to have limits. In the Netherlands, great tits are on the decline because they can’t adjust their schedules to temperature, according to a 2005 study.

One possible explanation for the difference between the populations is that winters in the Netherlands remain cold longer, providing little early warning that would allow the birds to adjust their egg laying, Charmantier said.

Walter Jetz, a UC San Diego ecologist who was not involved in the research, cautioned that the new analysis offers little assurance that the great tits of England will endure the accelerating temperature rise predicted over the next 50 years.

He added that the flexibility of the English birds may not exist in the tropics, home to 80% of bird species.

Because temperatures near the equator vary little season to season and year to year, the birds there may never have evolved the ability to adjust their breeding schedules.

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alan.zarembo@latimes.com