Climate change and El Nino a threat to song birds, study says

WASHINGTON - Climate change and a possible increase in El Nino events could seriously reduce the population of songbirds in North American forests, a study suggests.

Researchers at Dartmouth College and Tulane University analyzed the effect of natural climate cycles on the black-throated warbler and found that the migratory songbird did poorly in years when there was an El Nino cycle.

Dartmouth researcher T. Scott Sillett, first author of a study appearing Friday in the journal Science, said that El Nino climate cycles reduce the insect and caterpillar food supply for the warblers at both the bird's breeding grounds in New Hampshire and their winter home in Jamaica.

As a result, the small bird produces fewer young during the El Nino years. Conversely, during the La Nina years, the bird has more to eat and has better success in raising young, the researchers found.

El Nino refers to a warming of the usually chilled waters of the Pacific Ocean off the western coast of South America. The effect extends westward and causes a worldwide change in temperature and rainfall patterns.

La Nina has the opposite thermal effect, with some Pacific Currents becoming colder than normal. This, too, affects worldwide weather.

Sillett said there is concern that global warming, a general trend toward warmer temperatures worldwide, will increase the frequency of El Nino climate cycles. This could be very bad news, he said, for the warbler and other such small forest songbirds.

''If the El Nino cycle becomes stronger, it could increase the chances of having years when warbler survival and reproduction rates reach extreme lows, perhaps even approaching zero,'' said Sillett in a statement. He said strong El Nino cycles could ''elevate the risk of extinction'' for such birds.

Bernt-Erik Saether of the Norwegian University for Science and Technology wrote in Science that a ''frightening consequence'' of Sillett's study is that it shows ''how difficult it will be to reliably predict the effects of large scale regional climate change on ecological systems.''