An Early Spring Awakening
Every year, the arrival of spring seems to flip the switch of life: birds lay eggs, mammals rouse from hibernation and insects hatch. But appearances are deceptive. The season's unfolding is a complex and precisely timed sequence, with different species responding to specific cues. Today, especially in the temperate parts of the Northern Hemisphere, spring is arriving 10 to 14 days earlier than it did only 20 years ago. Scientists are finding that some species can adjust—and some can't.
After a winter spent in West Africa, pied flycatchers (Ficedula hypoleuca) migrate to northern Europe, Russia and Scandinavia, where they lay eggs and raise their young. Timing of this migration is triggered by day length.
But with warming, spring is coming earlier in the birds' destinations, and by the time they arrive, the caterpillars needed to feed the young birds have hatched and gone. Some flycatchers are laying their eggs earlier, but not soon enough to catch up to the caterpillars. In parts of the Netherlands, flycatcher numbers have plummeted 90 percent in 20 years. Experts say this mistiming is the reason.
Signal of Early Spring
In much of eastern North America, the calls of the frogs known as spring peepers (Pseudacris crucifer) are the first sound of spring. At the beginning of the 1900s that sound could first be heard in upstate New York around April 4th. By the 1990s, it was March 20th—a 2-week advancement of the season.