For most of the year magpies are not aggressive, but for four to six weeks during nesting
they will often defend their territory vigorously. People walking past may be seen as
'invaders' of the territory, prompting the magpies to fly low and fast over the person
clacking their bills as they pass overhead.

The experience of a magpie attack can be quite alarming, but it is usually only a warning.
Only occasionally will a bird actually strike the intruder on the head with
its beak or claws. If this unusual behaviour persists, there are ways of reducing
the risk of physical injury to humans.

If a magpie swoops at you:

Walk quickly and carefully away from the area, and avoid walking there when magpies are swooping.

Make a temporary sign to warn other people.

Magpies are less likely to swoop if you look at them. Try to keep an eye on the magpie, at the same time walking carefully away.

Alternatively, you can draw or sew a pair of eyes onto the back of a hat, and wear it when walking through the area.

You can also try wearing your sunglasses on the back of your head.

Wear a bicycle or skateboard helmet. Any sort of hat, even a hat made from an ice cream
container or cardboard box, will help protect you.

Carry an open umbrella, or a stick or small branch, above your head but do not swing it at
the magpie, as this will only provoke it to attack.

If you are riding a bicycle when the magpie swoops, get off the bicycle and wheel it quickly through the area.
Your bicycle helmet will protect your head, and you can attach a tall red safety flag to your bicycle or hold a stick or branch as a deterrent.

More.