Climate Change Could Wipe
Out Migrating Songbirds
By Cat Lazaroff

HANOVER, New Hampshire (ENS) - Global climate change could greatly reduce the number of songbirds migrating between North and South America. Researchers from Dartmouth College and Tulane University have shown that El Niño cycles, which could worsen due to global warming, reduce the birds' ability to survive and reproduce.
Many neotropical songbirds are marked by bright colors (Photo by William Paff, courtesy Cornell Lab of Ornithology(CLO))
El Niño is the pattern in the Pacific Ocean in which warmer water triggers droughts, floods and other weather disasters, as well as reducing the food supply for many marine species. Many scientists believe the recent increase in the frequency and strength of El Niño cycles can be attributed to rising global temperatures.
In the Caribbean, where many so-called neotropical songbirds spend their winters, El Niño leads to drier winter weather. That, in turn, reduces the available seeds and insects to feed birds, which must work hard to store up enough fat to migrate back to nesting grounds in North America.
The thinner, weaker birds that do survive the migration are less fertile, and less likely to reproduce successfully. "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 researcher T. Scott Sillett. In a paper published in today's issue of the journal "Science," Sillett and Richard Holmes, both of Dartmouth College, and Thomas 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 when Pacific Ocean waters are cooler than average.
The New Hampshire researchers studied black-throated blue warblers (Photo courtesy CLO)
Neotropical migrants such as the black-throated blue warbler breed in North America and spend the winters 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.
Part of the difficulty in determining the causes of songbird declines has been the inability of researchers to determine where particular birds went during migrations. Sillett, Holmes and Sherry were able to determine that their group of black-throated blue warblers always nests in New Hampshire and winters in Jamaica. "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 researchers found evidence that the climate changes associated with El Niño years diminished the birds' food supply, 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 much higher during La Niña years.
El Niño events can reduce the reproductive success of some neotropical migrants, like this blue-winged warbler (Photo by Betty Cottrille, courtesy CLO)
Poor reproductive success during El Niño summers resulted in fewer young birds arriving on the winter grounds in the fall.
"Adult survival and fecundity were lower in El Nino years and higher in La Nina years," the researchers wrote. "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."
"La Nina years, in contrast, tend to be wetter and thus would result in increased food availability and higher survival," the researchers reported.
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."
Ruby throated hummingbirds are also neotropical migrants, traveling vast distances each year (Photo courtesy Paul Conover)
Other species are affected as well, the researchers noted. The El Nino cycle has also impacted populations of seabirds, birds of prey such as hawks, monkeys and other primates, rodents and other animals. They pointed out that subtle changes linked to global warming, such as an alteration of the El Niño cycle, could have profound impacts on some species.
"Evidence is accumulating that bird populations are being affected by global warming associated with long term climate change," the researchers wrote.

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