HANOVER, N.H. -- As the United States government prepares to release a report predicting widespread climate changes due to global warming, a new study at Dartmouth College and Tulane University suggests that such climate changes could influence the number of songbirds populating the nation's woodlands and parks.
In a paper published in the June 16 issue of Science, researchers T. Scott Sillett and Richard T. Holmes, both of Dartmouth College, and Thomas W. Sherry, of Tulane University, analyzed 13 years of data on the black-throated blue warbler, a migratory songbird. They found that survival and reproduction rates for these birds were lower than average during El Nio years and higher during La Nia years.
"Our data show that the global climate cycle known as El Nio affects migratory birds both on their breeding grounds in North America and in their winter quarters in the tropics," said Sillett.
The researchers provide evidence that the climate changes associated with El Nio years diminished the birds' food supply, thereby causing low reproductive success in their New Hampshire breeding grounds and low survival rates among adults wintering in Jamaica. In contrast, reproductive success and adult survival were significantly higher during La Nia years.
Furthermore, poor reproductive success during El Nio summers resulted in fewer young birds arriving on the winter grounds in the fall. "Tracking individual songbirds throughout the year is very difficult, so being able to link summer and winter populations of a migratory species is a unique strength of our study," said Holmes.
The black-throated blue warbler is a member of the group of birds known as Neotropical migrants. These species breed in North America and spend the winter in Latin America and the Caribbean. In recent years scientists have become alarmed by population declines in many Neotropical migrant species.