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'The fear effect' threatens survival of our songbirds

Dec 09 2011
This song sparrow was part of a study that showed that simply hearing the calls and sounds of predators frightened the birds so much that they produced 40% fewer offspring. 

This song sparrow was part of a study that showed that simply hearing the calls and sounds of predators frightened the birds so much that they produced 40% fewer offspring.

Photograph by: Liana Zanette, .

Songbirds in the Gulf Islands are under stress and sometimes dying because they're frightened of predators while deer populations are booming because they have no predators to be afraid of, research by a University of Victoria biology professor shows.

The results of a decade-long study on the impact fear has on songbirds was published Thursday in the December issue of Science magazine and focused on songbirds on Portland Island and several other small Gulf Islands.

It was conducted by Michael Clinchy, a University of Victoria biology professor, and his wife, Liana Zanette, a biologist at the University of Western Ontario.

While the songbirds were the focus of the study, "our research has relevance to all sorts of birds and mammals," Clinchy said.

"We know the nestlings got fed less by frightened mothers," Clinchy said.

"This affects their stress physiology and brain development as nestlings. When they leave the nest, they have smaller brains and higher stress hormone levels, and that likely means their survival is affected as well."

Clinchy and Zanette found that Gulf Island songbirds protected from predators produced more young and had lower levels of stress hormones.

Half the sparrows in the study were protected from predators by nets and electric fences and heard recorded sounds of non-predatory animals for the duration of a 130-day breeding cycle. The other half were left exposed to threats and heard recorded sounds of various predators.

The study showed that sparrows exposed to predators and their sounds produced 40 per cent fewer offspring over the breeding season, Clinchy said.

"They laid fewer eggs, fewer of their eggs hatched and more of their nestlings starved to death," he said.

The findings are in line with earlier research into the re-introduction of wolves at Yellowstone National Park and the effect on elk populations, Clinchy said.

In both cases, the prey spent more time looking for predators and less time looking for food. These behaviours caused a drop in their reproduction rates.

"This is the first study to definitively and unambiguously isolate that fear effect," said Clinchy, adding that the 40 per cent drop in reproduction rate is likely conservative.

Having predators around not only leads to direct deaths of prey, but the fear adversely affects the birth rate and survival of young, Clinchy and Zanette found.

Clinchy said the abundance of deer in Greater Victoria and the lack of cougars, a major predator, "is an analogous situation to the elk in Yellowstone Park, where you have a lot of overbrowsing because the predators were eliminated."

The study results have "important implications for conservation and wildlife management because it suggests that the total impact of predators on prey populations will be underestimated if the effect of fear itself is not considered," Clinchy said.

"This means that the adverse effects of introduced predators are likely worse than previously imagined and the disturbance to native ecosystems due to the loss of native predators has likely been greater than we previously thought."


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