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 Niño years and higher during La Niña years.
"Our data show that the global climate cycle known as El Niño 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 Niño 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 Niña years.
Furthermore, poor reproductive success during El Niño 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.
"Most birds breeding in North American forests during the summer are Neotropical migrants. Besides their aesthetic value, they eat huge quantities of insects, and research has revealed that migratory songbirds can actually enhance tree growth by consuming leaf-eating caterpillars," Sillett said. Although the El Niño cycle is a natural phenomenon, many climatologists believe that global warming is escalating the cycle's frequency and severity. This could lead to stronger and more common El Niño and La Niñas cycles in the future.
"If the El Niño cycle becomes stronger, it could increase the chances of having years when warbler survival and reproduction rates reach extreme lows, perhaps even approaching zero," Sillett said. "More intense El Niño cycles could thus elevate the risk of extinction for Neotropical migrant species with small population sizes."
Sillett is a researcher and doctoral degree candidate in the Department of Biological Sciences at Dartmouth, where Holmes is as a professor of environmental biology. Sherry is a professor of ecology and evolutionary biology at Tulane.
Journal
Science