Sheer fear of predators, forces sparrows to cut down on laying eggs and hatching offspring


Song sparrow was the subject of a study on how fear of predators makes the bird lay lesser eggs (Pic. Courtesy Twitter/@KFlanaganphotos)

The equation of prey and predator has been existing since time immemorial with the former always on alert to save itself. Yet a report in mentions a recent study that even without predators attacking its victims, its fear itself can have a significant influence on the prey!

In an experiment, Liana Zanette and Michael Clinchy from Ontario’s Western University along with a graduate student of the institution Marek Allen, found out that mere sound of predators can have serious repercussions on song sparrows’ population for generations.

What the study unveiled was startling.

The reproductive success rate of the sparrows dropped by 53 per cent while fewer eggs were laid and hatched and fewer hatchlings made it to adulthood. There was a sheer drop in the population of the sparrows.

Worse, those young sparrows who grew up remained in fear though they hadn’t been exposed to the sounds of the predators. They sang a lesser number of songs, had shorter lives, and had very few offspring.

It was not just one generation that fear affected but several suggesting that the transgenerational cost of fear could cut the sparrow population in half in just four years.

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This study formed a part of a long-term project to research on song sparrows of five small islands in British Columbia’s Gulf Islands National Park Reserve.

While studying the role of fear on the behaviour of these birds, it was ensured that they were at no risk of predation. In the study, speakers were put up near the nests of these birds to convince them that the predators were nearby as the mixed tape of diverse sounds of sparrow eaters like crows, hawks, racoons and owls was played.

With the aim to make sure that the sparrows were reacting to the calls of the predators, speakers were placed near the nests of other sparrows which broadcast sounds of seals, ducks and frogs – all non-threatening sounds. These sounds bore resemblance to the vocal characteristics of the predators like for example, instead of raven’s caw, the birds heard the friendlier yet harsh honk of a Canada goose.

While these sounds were being played, it was assured that no harm came to these birds or their nests and eggs by electric fencing and overhead netting. While the sparrows could pass these defences, the predators like crows and racoons couldn’t.

The sparrows were subjected to these fearful sounds for four days while for another four no sound was broadcast. This was done for four months. The researchers then followed up each egg from the hatching stage to the point when the bird left the nest and was either dead or joined the breeding pool the next year. For three breeding seasons this experiment was done while the young ones were observed for two years of adulthood.

Throwing light on the findings of this research, behavioural ecologist of Los Angeles’ University of California, Dan Blumstein said: “They’ve shown the sound of fear alone cannot only affect individual behaviour, and individual fitness, but actually population trajectory.”

Blumstein, who in 2020 published a book on fear, even though was not a part of the study, visited the study sites. Talking about Zanette and Clinchy he remarked: “Their setup is really extraordinary.”

Song sparrow (Pic. Courtesy Twitter/@wmjohnson1)

Zanette observed that the research confirmed that “your early rearing condition leaves an imprint on you for your entire life.”

Dread among animals, including birds and mammals makes them eat less, keeping them busy watching for danger while it also allows less time for parents to feed the young.

Zanette’s team had in previous research discovered that restrictions in terms of food leads to smaller brains which hampers learning of songs and those birds whose repertoire is small don’t live long!