Life can be tough, even for birds of prey.

A lone barn owl found itself outnumbered more than a dozen to one on a crisp Perth morning at Scarborough Beach.

It swooped and soared as it was attacked by a murder of crows (Australian ravens) and a few magpies intent on running it out of the area.

Bird expert Dr Mike Bamford said the behaviour was known as "mobbing", where birds that fear they might be on the menu of a nearby predator turn the tables by going on the offensive, in force.

"Owls are particularly susceptible to this because they're easily recognised as a bird of prey, and they're not as dangerous as a falcon or an eagle or a hawk, so they're easy game to pick on," Dr Bamford said.

A barn owl is a night hunter that can snatch raven chicks from a nest, presenting a real threat.

The night hunter flies slowly during the day, making it a target. ( ABC News: Andrew O'Connor )

By day, the owl becomes a relatively easy target. Flying slowly, and unable to take advantage of its night vision or the cover of darkness, the owl is at a distinct disadvantage.

Dr Bamford said there were several theories why ravens would mob the owl.

"The main theory is they don't like the bird of prey. They're trying to make it move on," he said.

Dr Bamford said mobbing is both instinctive and opportunistic.

The instinct is evident when even tiny birds, like willy wag tails and honey eaters, mob a much larger bird, like a raven, to defend their nest.

The birds will often gang together to mob a predator. ( ABC News: Andrew O'Connor )

The opportunism becomes apparent when a predator arrives, and the mobbing birds seek security in numbers.

"The potential prey birds will often gang together to mob a predator. They often have very similar calls, a very strident sort of 'here's one, let's get this one'," he said.

"You'll see ravens, magpies, and honey eaters all mobbing a barn owl in the sky, so quite small birds and big birds as well, ganging up together against a barn owl."

The one-sided aerial duel between the owl and ravens continued for several minutes over land and sea before the owl broke away from its pursuers, finding clear air and safety.

Dr Bamford said not every encounter has a happy ending.

"I've once seen a barn owl killed on a playing field by magpies, but usually the owl just gets away with it," he said.