El Niño Punishes Migratory Birds

The population of a migratory bird species drops dramatically whenever El Niño, everybody's favorite source of strange weather, raises its head. If the population of endangered species fluctuates the same way, El Niño could speed their demise, researchers say.

El Niño, a warming of the tropical Pacific that occurs every few years, has been blamed for everything from flooding and mudslides in California to drought in America's heartland. But El Niño may have a subtler effect that spells trouble for already declining bird populations in North America. Ecologist Scott Sillett of Dartmouth College in Hanover, New Hampshire, and his colleagues have found that the black-throated blue warbler becomes more scarce during an El Niño, as unusual weather hits hard in both the birds' winter and summer homes.

At a New Hampshire breeding site, Sillett found fewer caterpillars, the warblers' favorite food, during the springs of El Niño years than during other springs. The shortages presumably arose because of changes in the weather, as El Niño generally makes winters warmer and drier than normal throughout the northern United States. With less food to eat, warbler fledglings began life smaller and weaker. Indeed, compared to other years, only 40% as many made it to their wintering grounds in Jamaica. But that was not the end of the warblers' troubles. El Niño also dried out Jamaica, which other researchers have shown cuts into the warblers' winter food supply. So on average only 44% of the birds survived to return the following year, compared with more than 60% at other times, Sillett and colleagues report in the 16 June issue of Science. "In El Niño years, you have a double whammy," Sillett says.

The damage from this two-fisted punch surprises Bernt-Erik Sæther, a zoologist at the Norwegian University for Science and Technology in Trondheim. "Most ecologists would have expected much less impact than we are now seeing," he says. While black-throated blue warbler numbers remain relatively strong, Sillett worries that these events could spell disaster for endangered birds because their numbers may be too low to allow them to recover fully after an El Niño.