Great Tit Birds Shift Mating Schedules Due to Warming

Matt Kaplan in London
for National Geographic News
May 8, 2008

Birds called great tits are adapting to global warming by altering their behavior in England, a decades-long study has found.

The common birds are found in gardens throughout most of Europe. They have a specific mating schedule linked both to warm temperatures and to the presence of caterpillars, which great tits feed to their young.

Researchers monitored a population of the birds in Wytham, near Oxford, for 47 years, looking carefully at when eggs were laid, which chicks survived, and what the conditions were at the time.

"We wanted to know how well great tits in Wytham could adjust their timing of breeding to the timing of food abundance … and how efficient this adjustment was for population adaptation to the increase in spring temperature," said lead study author Anne Charmantier of the University of Oxford.

The study appears in tomorrow's issue of the journal Science.

Early Birds

Because great tits rely on recurring annual conditions to rear their young, scientists had assumed that the birds instinctively lay their eggs at the same time every year—just before caterpillars become abundant.

Birds that laid eggs on their typical schedule would have a difficult time raising their young as warm temperatures—and caterpillars—arrived earlier, the researchers hypothesized.

Caterpillars would not be as plentiful when the eggs finally hatched a few weeks later.

Such a situation theoretically would lead to a steady decline in birds that stick to the old schedule, because their chick-rearing record would be poor, and would drive the population to have more birds that lay their eggs sooner—a classic case of natural selection.

(Related: "Early Birds: Is Warming Changing U.K. Breeding Season?" [June 3, 2002].)

Charmantier and her colleagues found that the Wytham population moved its egg-laying schedule forward 14 days to synchronize with the earlier caterpillar activity—without a lot of chick deaths.

Continued on Next Page >>


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