No early birds: migrators can't catch advancing caterpillars.

Pied flycatcher numbers are crashing in places where climate change has knocked the birds' spring migration out of sync with the food bonanza on the breeding ground.

That's the conclusion from a study of nine breeding areas in the Netherlands, says Christiaan Both of the Netherlands Institute of Ecology in Heteren. Each spring, the pied flycatchers arrive there after traveling 4,500 kilometers from Africa, cued mostly by day length.

Unfortunately for these birds, late-April weather in the Netherlands has warmed some 4[degrees]C during the past 20 years. In some places, that warmth has moved up the timing of a short-lived banquet of caterpillars, which adult flycatchers feed to their ravenous chicks. The flycatcher numbers in those locations have dropped by about 90 percent, Both and his colleagues report in the May 4 Nature.

"This is the first time that people have shown the population consequences of climate change [in a report] that also incorporates the mechanism,' says Both.

Dozens of other bird species make long-distance treks from Africa to Europe each spring. Earlier studies had indicated that numbers of other migrants have decreased as winter temperatures have risen.

The flycatchers offer an unusual opportunity to study the effects of global warming, Both says, because he has decades of data on their breeding success.

Previously, Both and his colleagues found that flycatchers tended to arrive at the same time each spring but, in recent years, were breeding more hurriedly than before. Over the past 2 decades, the birds shifted their nesting 10 days earlier. However, in those years, the big peak in caterpillar abundance advanced by 16 days. The profusion of caterpillars lasts only 3 weeks, so a 6-day mismatch can matter to the birds, says Both.

To study the effects of this timing, he and his colleagues examined sites across a range of more than 100 km. In some of the forests, leaves don't sprout early, and the burst of caterpillars that munch on them hasn't advanced. Both speculates that poor, sandy soil there may prevent early leafing out.

Both's team tracked caterpillar abundance through one spring by monitoring the insects' droppings under trees. The researchers used their previously collected data to work backward through the years. For example, they identified late-caterpillar areas for some years by seeing where European birds called great tits had raised second clutches of eggs.

Both and his colleagues found a correlation between declines in flycatcher numbers and the timing of the peak in food for their chicks. In the forests where the caterpillar peak was latest, flycatcher numbers had dropped by only 10 percent.

The scenario of migration miscues "very likely could apply to other birds," says ornithologist David Winkler of Cornell University. He notes that many European migrants, including the flycatcher, winter south of the Sahara Desert, so far from their breeding grounds that they lack information about spring weather there.

Ornithologist Scott Sillett calls the mismatch between migration itineraries and food availability "a very plausible mechanism" for climate effects on migrants. Based at the Smithsonian Institution's Migratory Bird Center in Washington, D.C., he tracks black-throated blue warblers.

He suspects that migration miscues might affect northern forests in North America. In summer, most of the birds there are migrants, some from South America, he says.