Great Tit Birds Shift Mating Schedules Due to Warming

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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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