How fear alone can impact birds and reduce population growth

“The only thing we have to fear is fear itself,” Franklin Roosevelt said during his inaugural speech in 1933. It has become his most famous statement.

Eleanor his wife, questioned about his origins, declared that Roosevelt had read the work of Henry Thoreau. “Nothing is more to be feared than fear”, was an entry in the diary of the great naturalist-philosopher.

But is fear to be feared? The results of a Canadian study, which has just been published, suggest yes, at least if you are a bird.

Predators have a direct impact on their prey, but do they also affect them in less obvious ways? Could stress, induced by the simple presence of predators, have an additional impact on wild creatures? If yes, how significant is the threat?


In a just-published article, Marek Allen and researchers at the University of Western Ontario address this question. They measured the effects of fear on a population of wild song sparrows. These distant relatives of our own house sparrow frequent bushes, gardens and roadside hedges all over North America.

While not everyone approves of subjecting wild creatures to artificially induced stress, Allen and his colleagues decided to put “fear of God” into song sparrows and record its effects on them. They chose sparrows living on five small coastal islands in British Columbia for the experiment. Some song sparrows migrate, but these islanders are year-round residents.

The team delivered “predator vocalizations at high but natural rates” to the sparrows for three breeding seasons. Calls were transmitted at 54 bird territories, while non-predatory bird vocalizations were transmitted at 51 sites.

Electric fences and netting ensured that each nest was protected from predators, so variations in nesting performance would be due to broadcast calls and not other hazards. Each nest was visited daily. Video cameras provided constant surveillance.

Both adults and young sparrows were given color-coded paw rings. Radio beacons tracked the movements of 151 young sparrows from their flight. Song “is predictive of survival to adulthood”, so the number of songs produced by each of the 24 “recruits” to the breeding population was recorded.

The fate of each of the 564 eggs and 507 fledglings has been determined “with certainty”.


The fear, the results showed, “significantly reduced population growth rates.” The numbers were to “halve” in just five years. Parent birds hearing predator calls produced 53% fewer recruits to the adult breeding population than those hearing non-predator calls.

Fear, the researchers conclude, “can constitute a very considerable part of the total impact of predators.” The fear factor has implications for all stakeholders.

Predators should keep the profile as low as possible, to “avoid spooking the horses”; ambush and stealth hunting methods are required. Vulnerable prey, on the other hand, must be hyper-vigilant to survive; “discretion is the best part of bravery” for them.

Zoologists who model natural populations must not only record the number of victims that predators kill and the production of young to compensate for the losses. These reveal only part of the true picture. Was Roosevelt wrong? do we, in fact, have much to fear from fear itself?

  • Mark Allen et al. Fear of predators in wildlife populations reduces population growth over generations. Proceedings of the National Academy of Sciences. 2022.
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