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.

The fact that few dead chicks were found meant that the tits were not making the mistake of laying on the old schedule as temperatures changed, the researchers say.

In other words, rather than being picked off by natural selection, the birds were surviving by shifting their behavior—adjusting their breeding schedule each year to coincide with caterpillar abundance, the scientists say.

(Related: "Global Warming Is Spurring Evolution, Study Says" [June 8, 2006].)

Striking Differences

Dan Nussey, an evolutionary biologist at the University of Edinburgh, Scotland, was not involved with the study.

"What is really striking here is the difference between this population and a population of great tits living in Holland, where springs are warming but the average bird breeding times are just not changing fast enough to keep up," he said.

"Same species, similar habitat, and yet quite a different picture. This study suggests that we cannot generalize about the effects of global warming."

Marcel Visser, an evolutionary ecologist at the Netherlands Institute of Ecology, was also not involved with the study. But he has a different opinion about why the two populations are responding to climate change differently.

In the United Kingdom, springs are getting warmer earlier. In the Netherlands, though, it's the late part of spring—after the birds have laid eggs—that is increasing in temperature.

"This is a big difference," he said. "In the U.K. the birds lay eggs as the weather warms and have caterpillars available to feed their young.

"In the Netherlands the birds lay their eggs when the weather is the same as it always was, but because the temperatures in late spring are much warmer, the [temperatures] accelerate insect development, and caterpillars are not available when it is time to feed the young," he added.

"The U.K. birds are just lucky that the old rule of laying eggs when temperatures get warmer still works for them. What is being observed in this new study is probably an exceptional situation."

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