暖化腳步快 長途遷徙的候鳥追趕不及

Many bird species migrating to Massachusetts from points south are arriving earlier each spring as temperatures warm along the east coast of the United States. Some of those species migrate thousands of miles from South America, but a new study shows that the farther the birds travel, the less likely they are to keep pace with the rapidly changing climate. Massachusetts scientists have learned that being slow to change in response to warming temperatures could have serious repercussions for long-distance migrant birds.

Researchers at Boston University and the Manomet Center for Conservation Sciences analyzed changes in the timing of spring migrations of 32 species of birds along the coast of eastern Massachusetts since 1970. They gathered this data by capturing birds in mist nets, attaching bands to their legs, and then releasing them. Manomet center has been banding birds since 1966 and is considered one of North America's oldest and most extensive landbird banding programs.

Their findings, published in the journal "Global Change Biology," show that eight out of 32 bird species are passing by Cape Cod earlier on their annual trek north than they were 38 years ago. The scientists attribute this change to the warming climate. Temperatures in eastern Massachusetts have risen by 2.7 degrees Fahrenheit since 1970.

Co-author Abraham Miller-Rushing, a doctoral student in biology at Boston University, has said, "Changes in the timing of phenological events like the flowering of plants and the arrival of migratory birds are among the most sensitive indicators of global warming's effect on biological systems."

Species such as the swamp sparrow that winter in the southern United States are generally keeping pace with warming temperatures and earlier leafing of trees. The study shows these species migrate earlier when temperatures are warm and later when spring is cool. Yet the migration times of birds that winter further south, like the great crested flycatcher, which spends its winters in South America are not changing, despite the warming temperatures in New England, researchers found.

There appears to be good reason for the difference between the short-distance and long-distance migrants.

Because temperatures are linked along much of the East Coast of the United States - an early spring in North Carolina is generally an early spring in Massachusetts - the short-distance migrants can gain insight into when it will be warm further north. They can follow the flush of leaves and insects all the way to their breeding grounds each year.

Long-distance migrants do not have any good cue for whether it will be an early or late spring on the northern stretches of their migrations. Weather in South America has little to do with weather in New England.

It appears that the short-distance migrants are keeping pace with this changing environment. However, long-distance migrants are being left behind; as temperatures continue to warm, they will probably experience environments increasingly different from the ones for which they are adapted, the scientists predict.

The inability of some birds to adapt to rapid climate change may be an important factor in some of the declines among songbird populations that have been documented in recent years.

Naturalist writer Henry David Thoreau ventured into the woods near Concord, Massachusetts more than 150 years ago to "see if I could not learn what it had to teach me."

In a related study, Miller-Rushing and co-author Richard Primack, a professor of biology at Boston University, are using Thoreau's notations of plant flowering cycles and bird migration patterns as a basis for research into the local effects of global climate change.

Primack says, "We hear about the effects of global climate change on hurricane systems, the Gulf Stream, the melting of glaciers. But a lot of this information seems very far away. We want to use Massachusetts, and Concord in particular, as a case study which demonstrates that global warming is happening, and it can't be ignored."