I'm very superstitious... I never shout at magpies, walk under ladders or put my shoes on the table. . . . Norman Cook

A magpie will only defend its nest within a ‘defense zone’. For pedestrians, this is usually an area within 100 meters of the nest but can be much larger for cyclists.
Almost all swoops on people are carried out by male magpies defending their eggs and chicks, which are in the nest for about six weeks from mid-August to the end of September.
Funny how Magpies are like people.
Image by Mezza ... Magpie at Hungry Head