Fear of predators means sparrows struggle to raise chicks to adulthood

Sparrows on a collection of Canadian islands change their behaviour when they fear predators are nearby, and an experiment shows this reduces their ability to raise their offspring to adulthood

By Luke Taylor

The fear of predators alone is enough to halve a bird population in just four years, a study in Canada suggests. 

Predators slow the population growth of their prey by killing and eating them, but how their presence affects the behaviour of their prey is little understood. 

To simulate the presence of natural predators, Liana Zanette at Western University in Ontario and colleagues hung speakers from trees in 104 territories of wild song sparrows across five islands in British Columbia, Canada. 

In 51 of the territories, the researchers used the speakers to expose the sparrows to the vocal calls of their predators, including hawks, raccoons and ravens. In the other 53 territories, the speakers were used to play the vocal calls of harmless species, such as geese. The experiment continued over the course of three breeding seasons.

The speakers were regularly moved, and the audio recordings of the predator or harmless species changed slightly to prevent the sparrows becoming accustomed to the audio recordings. What’s more, all of the sparrows’ nesting sites were protected from real predators with electric fences and nets with holes large enough for them to pass through, but too small for hawks and ravens to navigate.

The sparrows that heard the predator sounds managed, on average, to raise 53 per cent fewer offspring to a reproductive age than sparrows that heard recordings of harmless species.

Previous studies have found that the fear of predators reduces the number of eggs that birds lay, but didn’t study whether the birds experience any problems raising their chicks. When all factors were considered – from the number of eggs laid, to how many hatched and chicks’ development – the researchers project that a local sparrow population will halve in size in four years as a result of the presence – or perceived presence – of predators.

“These reductions occur at every stage of life and there’s a huge cumulative effect of fear on the number of offspring that are available to recruit into the breeding population,” says Zanette. 

This is partly because prey species must spend more time watching for predators, reducing the time available to forage for food, says Zanette. 

With less energy from food, the sparrows are restricted in the number of eggs they can afford to lay and the number of chicks they can afford to raise. Even chicks that do survive may experience side effects from a lack of food in the nest: studies suggest birds learn fewer songs when they have less food as chicks, probably due to the impact on brain development. 

All animals, from insects to elephants, exhibit fear responses to predators, so would be likely to also see a slowing in population growth, says Zanette, although birds and mammals may be affected more due to the importance of parental care in their development.

The impact that predators have by causing fear adds to the growing evidence that the loss of predators significantly affects entire ecosystems. 

“This really emphasises that predators are a keystone species that can be a powerful force through fear,” says Zanette.

Journal reference: PNAS, DOI: 10.1073/pnas.2112404119

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