If there's one thing a rainforest bird shouldn’t mind, it's rain. But a new study finds that torrential downpours stress out the white-ruffed manakin—and that may be why it and other tropical birds migrate up and down slopes.
Ecologists have long thought that tropical birds that change elevation during the year are following patches of food. But food availability accounts for only some bird movements, not whole migrations, says Alice Boyle, an ecologist at the University of Western Ontario in Canada, and she wanted to find a better explanation.
Boyle studies white-ruffed manakins, chickadee-sized birds that eat fruit. They can't go more than a few hours without a meal. At her study site in Costa Rica, some white-ruffed manakins stay at higher elevation all year, whereas others move lower down during the rainy season. Boyle noticed that males—which are smaller than females and may have to eat more often—are more likely to migrate. And she thought about the fact that it rains more heavily at higher elevations. "I put all this together and thought, 'It has something to do with storms,' " she says.
Of course, rainforest animals can function in the rain. But birds stop flying during downpours, which often come with winter storms. Boyle wondered whether rain cuts down on foraging time so much that small birds just can't get enough food at higher elevations.
To test this idea, Boyle and her colleagues set up nets to catch birds at two sites, one at high elevation and one at low elevation, from mid-October to late December. It was nice out for the first month and a half, and then in mid-November an intense storm arrived that lasted for 10 days. Afterward, the white-ruffed manakins appeared at lower-elevation sites, as did other species that are known to migrate to lower altitudes. That was "the part that I didn't set out to look for but turned out to be just a beautiful, natural experiment," Boyle says.
Boyle and her team took blood samples from netted birds. They found that levels of a stress hormone were higher on rainy days, as were levels of a chemical that indicated the birds were burning stored fat—a sign that they weren't getting enough food. "These rainstorms have really strong effects, both behaviorally and physiologically, in ways that nobody knew before," she says. She says that the stress of rain may prompt birds—particularly small birds that have to eat constantly to survive—to fly downhill to drier ground. Other animals might move as well, she says. "If rain is going to impede the foraging of a small bird, it's probably going to hit a butterfly even worse." Her research will appear online tomorrow in the Proceedings of the Royal Society B.
Although this sounds like a reasonable explanation for white-ruffed manakins, it may not be true for every kind of bird that changes altitude during the year, says Chris Merkord, an ecologist and Ph.D. student at the University of Missouri, Columbia. "There could be a couple of thousand species that are altitudinal migrants around the world," he says. "My guess is that they're not all doing it for the same reason."
The research shows nicely that storms are stressful, says Doug Levey, an ecologist at the University of Florida, Gainesville. "I think that the idea is good: Tropical storms are potentially perilous for animals that have to maintain a high metabolic rate." But he doesn't think this study proves that storms drive migration, because it only covered a few months and didn't follow individual birds.