Nothing but fear itself can actually be dangerous for nesting birds.
Song sparrows protected from attack but subjected to recordings of predator yowls and leaf-crunching approach noises raised 40 percent fewer offspring in a year compared with neighbors living amid innocuous noises, says population ecologist Liana Zanette of the University of Western Ontario in Canada. Predators do not need to kill a single prey to have a big effect, she says.
Scary noises, broadcast where the sparrows nested in the wild, took a toll throughout the breeding season, Zanette and her colleagues report in the Dec. 9 Science. The alarmed sparrows laid fewer eggs to begin with, and they proved such skittish and cautious parents that they reared a smaller percentage of hatchlings than neighbors did.
Biologists have tended to focus on the direct effects of predators killing prey, says evolutionary ecologist Thomas Martin of the U.S. Geological Survey in Missoula, Mont., who was not part of the sparrow research. This new study, he says, suggests theorists have underestimated the impact of predators.
“Predators shape everything,” Zanette says. Wolves that eat elk give more plants a chance to survive, which in turn changes which other creatures thrive.
Previous work, including Zanette’s, suggested that fear of predators could change bird egg number or size. Yet separating the effects of fear from those of actual predator attacks took years of preparation, she says.
Working on small, uninhabited islands off the western coast of Canada, Zanette and her colleagues set out cameras to identify which predators were feasting on the vulnerable, open-cup nests of song sparrows there. Researchers then devised obstacles to those predators, fencing 24 nest sites to keep away raccoons and building little teepees swathed in netting to keep out ravens and owls but allow tiny sparrows to dart through.
For half the birds, researchers played recordings of some predator sound every few minutes in random order around the clock for four days at a time, allowing four-day sound vacations between broadcast stints to keep sparrows from getting used to the recordings. The sounds stretched over a whole breeding season of raising two batches of young per family. The other half of the sparrows heard unalarming broadcasts, such as recorded goose honks and loon wails.
Before this, no one had tracked effects of predator sounds during an entire bird breeding season, Martin says. And, he adds, the study neatly isolates the effects of predator risk.
What ornithologist Sönke Eggers of the University of Agricultural Sciences in Uppsala, Sweden would like to know now is whether those chicks lost to indirect effects would have been eaten anyway in the real world. Zanette predicts that fear does take a toll on real-world nests beyond direct attacks from predators.
