El Nino hits birds north and south, study finds
Date: 19-Jun-00
Country: USA
Author: Maggie Fox, Health and Science Correspondent
El Nino makes for drier weather in the winter in the Caribbean, the researchers
said, leading in turn to thinner, less-fertile birds. Other experts said the
study, published in Science, showed that even subtle changes linked to global
warming have profound effects for animal populations.
T. Scott Sillett of Dartmouth College in New Hampshire and colleagues at Tulane
University in New Orleans studied black-throated blue warblers, which breed in
eastern North American forests and spend their winters in the Greater Antilles
islands of the Caribbean.
Sillett's teams watched the warblers at their summer homes in New Hampshire and
their winter nests in Jamaica.
They compared their population swings to the cycle of El Nino, the Pacific Ocean
current that recurs every few years, bringing unusually warm water to a large
area off the coast of Peru. It affects weather all over the world.
They also watched a counter-cycle that has come to be called La Nina.
"Adult survival and fecundity (fertility) were lower in El Nino years and higher
in La Nina years," they wrote in their report, published in the journal Science.
"During El Nino years in Jamaica, reduced rainfall probably leads to a decreased
amount of food available for warblers in the winter dry season and, hence, to
lower survival," they added.
Thinner birds would not breed as well in summer.
"La Nina years, in contrast, tend to be wetter and thus would result in increased
food availability and higher survival." They noted reports that show the El Nino
cycle has affected populations of seabirds, raptors such as hawks, primates,
rodents and other animals.
"Evidence is accumulating that bird populations are being affected by global
warming associated with long-term climate change," they wrote.
"Global warming could also be increasing the severity of ENSO (El Nino Southern
Oscillation) events. If this is true, we predict that variance in demographic
rates of migratory bird populations will become amplified, leading to elevated
extinction risk, especially for small populations."
Centuries ago El Nino occurred every two to 15 years, but recent research shows
the pattern has become more frequent.
The researchers said the need to understand what affected bird populations was
becoming pressing as migratory songbirds are disappearing at alarming rates.
Bernt-Erik Saether of the Norwegian University for Science and Technology said
many studies had shown climate affects birds at their over-wintering points.
"The study of Sillett et al provides the first evidence that the demographics of
a migrant bird, the black-throated blue warbler, may be strongly influenced by
large-scale climate changes affecting not only survival in southern wintering
grounds but also reproductive performance in northern breeding areas," he wrote
in a commentary published in Science.
