MICHELE NORRIS, Host:
Now on to a report about one of the ways the greenhouse effect may be altering the world around us. As temperatures rise, spring appears to be arriving earlier in many parts of the world. That might sound like a good thing to you, but for migrating birds it could be deadly. NPR's John Nielsen reports
JOHN NIELSEN: Songbirds that fly north out of the tropics every spring have taken lots of manmade hits in recent decades. Forests at both ends of their migration routes have been replaced by farms and cities. Millions of migrating birds have also been killed by house cats and unseen windows. But Paul Strode, a biologist affiliated with the University of Colorado, says there's always been a big reward waiting for the birds that make it all the way to their northern nesting grounds.
PAUL STRODE: Insects are peaking at abundance and caterpillars time their emergence from eggs to match the emergence of leaves on the trees. The leaves that they eat.
NIELSEN: It use to be that the birds arrived precisely when the caterpillars were at their peak. That made it easy for the birds to fatten themselves up, lay their eggs and feed their young. But now there's a problem, Strode says. In the forests he's been studying in the Midwest, the leaves are sprouting much sooner than they use to. The caterpillars have followed suit, which means they are much less common when they birds finally do arrive. Strode says ornithologists have wondered, could these earlier spring times be a reason why some of the most loved birds in the country are now so hard to find?
STRODE: Birds like the Magnolia Warbler, the Black Threaded Green Warbler, the Oven Bird, which is also a wood warbler. Gregarious in the spring, incredible and varied songs the males do. They're incredibly colorful, so they're very popular for birders.
NIELSEN: Until this week there'd been little evidence that linked the decline of birds like these to advancing northern spring times, but now a group of scientists in the Netherlands says it sees a connection. They've been studying the near disappearance of a European songbird called the Pied Flycatcher, which loves to eat caterpillars.
Christiaan Both, an ornithologist with the Netherlands Institute of Ecology, says it's no coincidence that spring now arrives in the Flycatcher's forest 16 days earlier than it did 20 years ago. As a result, young birds are malnourished, he says, and not ready to migrate South in the fall.
CHRISTIAAN BOTH: Those birds, they have to migrate to Africa so they really need to hatch in prime condition. And otherwise, they can't make this whole journey.
NIELSEN: Both says the Flycatcher study is far from definitive. Other factors could've forced this change such as the arrival of new predators. He also says its possible that some day the Pied Flycatcher will adapt to earlier spring times by shifting the timing of its migration. Paul Strode says migrating birds have made those adjustments before during major climate shifts that occurred long ago. But he also says there's a crucial difference between what happened then, and what may be happening now. Back then, birds had thousands of years to adapt to shifting seasons, he says.
STRODE: But with the current predictions of climate change, we may be giving them 50 years to make the same adjustment that they made over 2,000, and that is alarming and shocking.
NIELSEN: A letter describing the study of Pied Flycatcher appears in the current edition of Nature. John Nielsen, NPR News, Washington. Transcript provided by NPR, Copyright NPR.
