Ask an ornithologist and you'll learn that raising a clutch of baby birds ought to depend on environmental conditions near the nest. But it turns out that what happens at their wintering grounds can be even more important. Migratory songbirds of the Pacific Northwest, researchers have learned, produce up to three times as many offspring when El Niño greens up their wintering grounds down south.
To investigate the impacts of climate fluctuations on Northwestern migratory bird populations, ecologist M. Philip Nott and colleagues at the Institute for Bird Populations in Point Reyes Station, California, captured birds in "mist" nets at 36 stations in forests of Oregon and Washington between 1992 and 2000. They tabulated the number of juvenile birds versus adults for 34 species breeding near the stations. Combining their numbers with climate data, they found that during the El Niño years (1992-93 and 1997-98), which bring cooler, wetter conditions to the birds' Central American wintering sites, juveniles born that season outnumbered adults 2 or 3 to 1. In the intervening La Niña years, that ratio was reversed: Adults outnumbered juveniles roughly 2 to 1, the researchers report in the July-August issue of Global Ecology and Biogeography.
They also discovered that the warm phase of the climate cycle called the North Atlantic Oscillation appears to similarly influence reproduction rates in birds that overwinter in Western coastal states. "Surprisingly, breeding success in both groups of birds correlates most strongly with late winter and early spring conditions on their wintering grounds, months before they begin breeding," says Nott. The birds likely benefit when milder temperatures and more rain during the warm phases of the climate cycles make food more abundant in their winter quarters, Nott says. Presumably the birds get more fit before the rigors of migrating and parenthood. Also, in the case of El Niño, climate data indicate that stronger northerly tail winds prevail in winters after an El Niño, which probably assist long-distance migration.
The research is unique in linking local population changes for multiple bird species to broad-scale climate changes, says Beatrice Van Horne, a wildlife researcher at the U.S. Forest Service in Arlington, Virginia: "Things that happen at a large scale are hard to see because local variance often confuses us. This work offers an important piece in the puzzle of understanding global-scale patterns."
The Institute for Bird Populations