How songbirds decide when to round up owls

Flight is not the only way songbirds can protect themselves from predators. Many songbird species are known to engage in stalking, where they aggressively crowd around a bird of prey, flying rapidly while performing stereotypical movements and loud vocalizations. Mobbing is risky for both parties: birds of prey have been observed attacking their attackers, while cases are known of birds of prey being injured by harassing songbirds.

Now, biologists from Oregon State University have shown that songbirds can determine when the risk of predation by a common predator is highest, based on season and geography. In response, they increase the frequency of harassing behavior. When this risk is minimal, they are more likely to avoid or ignore the predator, in this case the Northern Pygmy Owl. The results are published in Frontiers of ecology and evolution.

“Harassment must be energy intensive, as we find it to be rare in winter when food is scarce but there are still plenty of songbirds around,” said second author W. Douglas Robinson of Oregon State University.

“In addition to this effect, the likelihood of harassment also increased as the number of songbirds present increased, diluting the risk for each gangster. This allows songbirds to assess when the risk of predation by the Northern Pygmy Owl is highest and when bird numbers are safe.

Mammal and bird ambush predator

Robinson and her graduate student, Madeleine Scott, the study’s first author, studied the harassment of little owls in western Oregon near the town of Corvallis and in the nearby mountains. The Northern Pygmy Owl is a small diurnal owl of western North America that usually attacks small mammals and songbirds by ambushing them from a hidden location.

“The proportion of small birds to small mammals in the Northern Pygmy Owl’s diet nearly doubles from spring to summer, making birds the primary food source in summer. This is likely due to the increasing availability of full fledged birds,” Scott said.

Mobbing caused by reading owl calls

Throughout 2020 and 2021, Scott and Robinson broadcast recordings of the Northern Pygmy Owl’s advertising call a total of 663 times, from 547 different locations, at altitudes between 80 and 1,200 meters, to provoke assaults. Each broadcast lasted one minute from a loudspeaker mounted on a 3.3 meter high pole. The species calls throughout the year (with periods of up to 60 minutes) to attract mates and establish territories. Before and after each reading, the authors recorded the number and species of songbirds present within 50 meters of the enclosure. They also noted whether the songbirds intensified their own vocalizations, moved within five meters of the speaker, or exhibited aggressive behavior towards them.

The Pacific Wren is one of the species that attacks Little Owls. Photo by Jeremy Gatten

Overall, bullying was observed in 8.1% of the trials. Harassment behavior peaked (seen in up to 23% of trials) in late summer and autumn, when the pygmy owls preyed primarily on young birds. The behavior was rare in winter and spring (1%), when owls prey mainly on small mammals. The likelihood of stalking also decreases with altitude, which correlates with lower owl density at higher altitudes.

As many as 24 species of songbirds engaged in stalking: the most common were chickadees, red-breasted nuthatches, Pacific wrens and dark-eyed juncos, all small species that feature on the owl menu. Larger species, such as thrushes and jays, have rarely been observed as prey for the Northern Pygmy Owl, and they rarely engage in harassing acts, although they generally attack larger owls.

Diluting the risk of moral harassment

On average, 12.8 songbirds were recorded near the enclosure during playback, and the likelihood of harassment increased as this number increased.

The authors conclude that songbirds tend to follow a rule of thumb: only appeal to the crowd if the threat to you is real; otherwise, go about your own business. But when the threat is real and the harassment is beneficial, only do so if there are enough other songbirds around to dilute the risk.

“Future research questions should investigate the impact of the energy cost of bullying on the frequency of the behavior. For example, examining seasonal food availability and supplementing with additional feeders could reveal how energetic considerations influence bullying behavior,” Scott said.

Thanks to Frontiers in Ecology and Evolution for providing this news.

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