Angry tweet: Songbirds mob predators by striking when time is right

Mobbing is risky for both parties as birds of prey may attack the swarm, so the smaller birds only fight at oppportune moments

The northern pygmy owl sometimes finds its prey fights back (W DOUGLAS ROBINSON)

Songbirds launch en-masse attacks on birds of prey instead of fleeing – but only fight when the threat of attack is highest.

Many songbird species are known to engage in mobbing, where they gather aggressively around a bird of prey to harass them with sound and movement.

Mobbing is risky for both parties as birds of prey may attack their mobbers, while the predator risks injuries from the massed prey.

Now biologists from Oregon State University have shown that songbirds can tell when the risk of predation by a common predator is highest, depending on season and geography.

In response, they increase the frequency of mobbing behaviour. When this risk is minimal, they are more likely to avoid or ignore the predator, in this case, the northern pygmy owl.

The authors conclude that songbirds tend to follow a rule of thumb: Only mob if the threat is real toward you; if not, go about your own business.

But when the threat is real and mobbing is beneficial, only do so if there are enough songbirds around to dilute the risk.

“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 second author Professor W Douglas Robinson from Oregon State University.

“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.”

These results are published in Frontiers in Ecology and Evolution.

Robinson and his graduate student, Madeleine Scott, the study’s first author, studied mobbing of northern pygmy owls (Glaucidium gnoma) in western Oregon, US, near the town of Corvallis and in the nearby Pacific Mountain Systems. Northern pygmy owls are a small, diurnal owls species of western North America, which usually attack small mammals and songbirds by ambushing them.

Scott said: “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.”