When confronted with predators, songbirds have more than just their fast and furious flight to protect themselves. Many perform a technique called mobbing, where large numbers of song birds aggressively gather around a bird of prey and rapidly fly while vocalizing loudly.
Mobbing does come with some risk, since birds of prey can still attack the mobbers. However, according to some new research, the songbirds do have an innate weapon for understanding these chances: they can use seasons and geography to tell when the risk from a common predator is highest so that they can increase mobbing behavior. If the threat is lower, they can ignore predators.
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The results of a study on how songbirds assess this risk when facing the northern pygmy owl is explained in a study published January 31 in the journal Frontiers in Ecology and Evolution.
“Mobbing must be energetically costly, because we find that it’s rare during winter, when food is scarce but there are still plenty of songbirds around,” said study co-author W. Douglas Robinson, a bird ecologist from Oregon State University (OSU), in a statement. “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.”
Northern pygmy owls are a small, day-time dwelling (diurnal) owl species found in western North America. They usually attack prey, such as songbirds and some small mammals, by ambushing them from a hidden location. The owls in this study were from 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,” said co-author and OSU graduate student Madeleine Scott, in a statement. “This is presumably because of the increasing availability of fledged offspring birds.”
In 2020 and 2021, the team played back recordings of a northern pygmy owl’s “advertising call” 663 times at 547 different locations of varying altitudes in order to provoke mobbing activity in the songbirds like Pacific wrens and others. They noted the number and species of songbirds that were present before and after each owl call and if the songbirds intensified their vocalizations, moved within 16 feet of the speaker, or showed any mobbing behavior in its direction.
Mobbing was observed in 8.1 percent of all trials. It peaked during the late summer and autumn when northern pygmy owls prey on young birds, when mobbing was behaved in 23 percent of the study’s trials.
Mobbing was rare (only one percent of the trials) in the winter and spring. This is when the owls primarily eat small mammals, like shrews and moles. They also found that the likelihood of mobbing decreased with altitude.
Of the 24 species of songbirds engaged in mobbing, chickadees were the most frequent mobbers, followed by red-breasted nuthatches, Pacific wrens, and dark-eyed juncos. These small species are all on the owl’s dinner menu.
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On average, 12.8 songbirds flocked nearby the speaker when the owl calls were played back, and the likelihood of birds mobbing increased as this number became greater.
According to the authors, songbirds tend to follow a standard rule: only mob if the threat is real and if not, go about your birdy business. When the threat is real, mobbing will be beneficial, but only if there are enough songbirds nearby 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 how energetic considerations influence mobbing behavior,” said Scott.
