Fleeing is not the only way in which songbirds protect themselves from predators. Many species are known to engage in mobbing too, a phenomenon where they gather aggressively around a predator, flying rapidly while producing loud vocalizations and stereotypic movements. Such a behavior is risky for both prey and predator, with some birds getting attacked and even injured by their mobbers.
Now, a team of researchers from Oregon State University (OSU) has found that songbirds can tell when the risk of predation by a common predator is higher, depending on season and geographical features, and increase the frequency of mobbing behavior. By contrast, when the risk is minimal, they are more likely to avoid or ignore the predator.
“Mobbing must be energetically costly, because we find that its rare during winter, when food is scarce but there are still plenty of songbirds around,” said study co-author W Douglas Robinson, an ornithologist at OSU.
“On top of this effect, the likelihood of mobbing also increased as the number of songbirds present increased, diluting the risk to each mobber. Thus, songbirds can assess when the risk of predation from northern pygmy owls is highest and when there is safety in numbers.”
The experts studied mobbing of northern pygmy owls (Glaucidium gnoma) – a small, diurnal own species that usually attacks small mammals and songbirds by ambushing them from a hidden location – in western Oregon.
“The proportion of small birds relative to small mammals in the diet of the northern pygmy owl almost doubles from spring to summer, making birds the primary food source in summer. This is presumably because of the increasing availability of fledged offspring birds,” explained study lead author Madeleine Scott, a graduate student in Fisheries and Wildlife Conservation at OSU.
In 2020 and 2021, the scientist played back hundreds of recordings of an advertising call of the northern pygmy owl at a variety of locations to provoke mobbing. Before and after each playback, they recorded the number of species of songbirds present within 50 meters from the speaker.
While overall, mobbing was observed in 8.1 percent of the trials, the behavior peaked (up to 23 percent) in late summer and autumn when the owls prey mostly on young birds, and was quite rare in spring and winter (one percent), when they usually hunt small mammals. Moreover, the probability of mobbing also decreased with altitude, correlating with the lower owl density at higher altitudes. Twenty-four species of songbirds engaged in mobbing, the most frequent ones being chickadees, red-breasted nuthatches, Pacific wrens, and dark-eyed juncos.
According to the researchers, songbirds only mob if the threat is real and there are enough songbirds around to dilute the risk.
“Future research questions should study how the energetic cost of mobbing impacts the frequency of the behavior. For example, examining seasonal food availability and supplementing with additional feeders could reveal the how energetic considerations influence mobbing behavior,” Scott concluded.
The study is published in the journal Frontiers in Ecology and Evolution.
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