When Franklin D. Roosevelt said we have nothing to fear but fear itself, he could have easily been talking about songbirds. A new study shows that the mere sound of predators reduces both the number and survival rate of songbird offspring, regardless of the true threat. The finding could have important implications for managing wildlife, not just for protecting songbirds but for a host of other species.
Ecologist Liana Zanette of the University of Western Ontario in London, Canada and her colleagues studied song sparrows (Melospiza melodia) on several of the small Gulf Islands in British Columbia. They surrounded nesting areas with netting and electric fencing to keep out predators and set up speakers broadcasting the sounds of other animals. In some areas, they played the sounds of predators, such as raccoons, hawks, and owls. In others, they played nonpredator sounds, including seals, geese, and hummingbirds. During the 130-day nesting season of the sparrows, the speakers broadcast sound every few minutes 24 hours a day in a 4-day-on-4-day-off cycle.
Female birds exposed to the sounds of predators showed drastic changes in behavior. They built nests in denser and thornier plants, spent more time watching for predators and less time collecting food, and produced fewer eggs—something that has been linked to lower food consumption in the past. Once their eggs hatched, the mothers provided less food to their nestlings—making fewer than eight feedings trips an hour, on average, as opposed to the standard 11, and only straying half the distance from the nest as usual to find food-and fewer babies survived. In all, the birds exposed to predator sounds produced 40% fewer fledglings than birds exposed to nonpredator sounds, the team reports online today in Science.
The finding has implications for species conservation, says ecologist and study co-author Michael Clinchy of the University of Victoria in Canada. For instance, catch-neuter-release programs to control feral cat populations in some cities operate under the assumption that if feral cats are well-fed by caretakers who provide food in city parks, they won't harm wildlife, such as birds. "But our results show that the mere presence of this introduced predator is enough to negatively impact native wildlife," Clinchy says. That's especially concerning because many feral cat programs are located in conservation parks.
The new finding, Clinchy says, could also help clear up a long-standing debate about the effectiveness of reintroducing wolves to Yellowstone National Park—an initiative that began in 1995 in an effort to restore Yellowstone's native flora and fauna. Proponents of the wolf reintroduction say that wolves, which once thrived in Yellowstone but went extinct in the 1920s, control the elk population, in turn allowing native plants and smaller animals to flourish. But critics say the decrease in the elk population since 1995—a 50% decline—can't be due to wolves because they don't kill and consume enough elk per year.
"Our results corroborate evidence that wolves are decreasing elk numbers," Clinchy says. "Not by killing but by scaring them." Frightened elk, he says, are known to behave similarly to the sparrows exposed to predator sounds—they spend time in safer places and consume less food, leading to decreased offspring. But this effect of predators on fear, and hence offspring, had never been quantified before, says Thomas Martin, a University of Montana, Missoula, ecologist who was not involved in the study.
Still, Martin says more work is needed before the results can be applied to animals besides songbirds. "They looked at a single species, and clearly this will differ across species and across habitat types," he says. "This now opens the door to other studies."