Before you've realised what has happened, you've either been lucky enough to escape with a close fly-by warning or have been left with a bleeding head.

The black and white bombers are back. It's magpie swooping season.

While it might mean a trip to the letterbox or trying to enter the school gate is navigated with sheer terror for a few weeks, the act of swooping by magpies means all is well in nature.

Like any tired and anxious parent bringing a newborn home to the nest, they are protective of their young want you to stay away from them.

A Charles Sturt expert explains why the birds behave in this territorial and seemingly aggressive manner, and the best strategies to avoid attacks.

Charles Sturt University (CSU) lecturer in Ornithology in the School of Environmental Sciences and researcher at the Institute for Land, Water and Society, Dr Melanie Massaro, says swooping - or mobbing behaviour - is not unique to Australian magpies.

Many birds, including terns, gulls, skuas and lapwings defend their nests and offspring by mobbing predators.

"It is likely that swooping behaviour in magpies evolved originally to defend their offspring from other predatory birds, such as currawongs or raptors, and tree-climbing reptiles and mammals. But nowadays, in areas where magpies and humans share spaces, some magpies swoop people," Dr Massaro explains.

Magpies defend territories all-year round, but they will only swoop humans when they are nesting. And, believe it or not, it is not their intention to hurt people; they simply want to encourage you - using their magpie style - to move further away from their nest.

A previous study found that only males swoop humans and they are most protective when they have big chicks in the nest. Another study tested whether males that swoop have higher testosterone levels than those that do not, but the researchers did not find any differences in behaviour.

"Many birds learn from their parents to recognise predators and how to respond appropriately to different predators, so it is possible that young magpies learn from their dads whether to swoop humans or not," Dr Massaro said.

Australian magpies can live up to 25 years, so if you encounter a swooping male during one breeding season, it is likely that he will still occupy the same territory the following year.

So what are the best strategies to avoid being swooped? The most simple and obvious tactic is to avoid the magpie's territory during the breeding season.

"If that is not possible, it is advisable to where a hat or helmet (bike riders) for protection from swooping. If you find yourself under attack, raise your arm over your head to protect your head and move as quickly as possible out of their territory. A magpie will stop swooping when you are out of its territory," Dr Massaro said.

"Magpies are very aware of people's eyes, and therefore they often attack from behind. A study conducted on masked lapwings found that fake eyes on the back of hats protected pedestrians from swooping attacks, but this little trick didn't work for cyclists.

"As a public safety act, councils that become aware of swooping magpies should put signs up to warn pedestrians and cyclists."

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