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Early Birds Miss the Worms

30 March 2001 7:00 pm
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Climate casualties. Changing climates may cause blue tits to breed out of sync with their food source.

Raising young takes time and burns energy. There's also the risk that a creature won't have enough food for itself or its young. Now a long-term study published in the 30 March issue of Science confirms the toll that breeding can take if nature doesn't cooperate. The paper suggests that global warming may catch some birds in a race against time, because they may end up breeding before food is plentiful.

Each spring since the 1970s, evolutionary ecologist Jacques Blondel of the Center of Functional and Evolutionary Ecology in Montpellier, France, has studied small birds called blue tits in the woods near his institute and in Corsica, an island 125 kilometers away. Typically, blue tits in the two locales breed in different habitats--evergreen oak trees in Corsica, and deciduous oaks around Montpellier. Yet some of the continental birds were atypical and nested in local evergreen oak forests, just like the ones in Corsica.

Four years ago, Blondel and Donald Thomas, a physiological ecologist at the University of Sherbrooke in Quebec, Canada, decided to look more closely at these odd ones out. By injecting isotopes, the researchers learned that the Montpellier birds breeding in evergreen oak forests use almost twice as much energy in rearing their young as the birds in Corsica do. The birds hopped about at a rate that they can't sustain for long.

The reason: bad timing on the part of the Montpellier birds, Thomas says. The Corsica blue tits breed in June, right when new leaves come out and leaf-munching caterpillars are most abundant. On the continent, most birds breed 3 weeks earlier, coinciding with the greening of the deciduous oaks and, again, an abundance of caterpillars. That puts the atypical population nesting in evergreen oaks near Montpellier at a disadvantage: Those trees are not budding, and caterpillars have not yet emerged in May. Out of sync with their food source, those birds end up having to work much harder. That may burn fat reserves, leaving the birds more vulnerable to starvation during the winter, Thomas suggests.

The new work "confirms what many people thought but were never able to show: that breeding too early has a fitness cost," says Marcel Visser of the Netherlands Institute of Ecology in Heteren. Thus, the work hints that as climate changes and the timing of the seasons shifts, says Blondel, "more and more populations of birds will become maladapted to breeding."

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