Bird Migration May be Affected by Warming Climate

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Spring bird migrations are seeing some birds returning earlier each spring as temperatures warm along the East Coast of the United States; the farther the journey the birds must take to reach their summer home, the less likely they are to keep pace with the rapidly changing climate.

The changes in the timing of the spring migrations of 32 species of birds along the coast of eastern Massachusetts since 1970 have been analyzed by scientists at Boston University and the Manomet Center for Conservation Sciences.

The research team at Manomet gathered this data by capturing birds in mist nets, attaching bands to their legs, and then releasing them. Their findings, which were published in Global Change Biology, show that eight out of 32 bird species are passing by Cape Cod significantly earlier on their annual trek north than they were 38 years ago. The reason is warming temperatures. Temperatures in eastern Massachusetts have risen by 1.5 degrees Celsius (2.7 degrees Fahrenheit) since 1970.

The swamp sparrow, for example, winters in the southern United States and is generally keeping pace with warming temperatures and earlier leafing of trees. They migrate earlier when temperatures are warm and later when spring is cool.

Birds that winter further south, like the great crested flycatcher, which spends its winters in South America, are slow to change, though. Their migration times are not changing, despite the warming temperatures in New England.

There appears to be good reason for the difference between the short- and long-distance migrants. Temperatures are linked along much of the East Coast of the United States, for example, an early spring in North Carolina is generally an early spring in Massachusetts and this means that he 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.

The long-distance migrants do not have any good cue to inform them 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. The long-distance migrant birds, being slow to change in response to warming temperatures, may face serious repercussions.

Plants are blooming earlier in Massachusetts than they did in the past. It appears that the short-distance migrants are keeping pace with this changing environment.

The 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.

Other researchers have already noted that some long-distance migrant birds returning from African wintering areas to breed in Europe are not keeping pace with their insect food supply.

Some of the declines among songbird populations that have been documented in recent years may be attributed to the inability of some