National Parks and Wildlife Service wildlife management officer Geoff Ross said the native birds would be ``at their most aggressive'' in coming weeks.

"At this time of year, magpies will swoop at whatever they see as a threat,'' Mr Ross said. "They don't generally swoop to make contact, only to scare you, although sometimes they misjudge the distance and end up running into you.''

He said the best way to avoid being swooped was to keep well away from magpie nests. "If you have to go through nest areas, keep your wits about you. Walking with an umbrella sometimes can keep them away,'' he said.

Mr Ross said magpies tended to swoop at fast-moving objects, such as postmen, dogs and cyclists.

Sutherland Shire Cycling Club John Peters president said he knew this fact all too well.

"I've been swooped at a few times over the years while riding,'' he said. "It's quite a shock when they make contact as it feels like someone has thrown a rock at you.''

He said at least one club member had been struck and injured by a swooping magpie this season.

"It can be very distracting for a cyclist and has the potential to cause accidents,'' he said.

Harry Berkery, of Gymea Bay, a club member since 1974, attached a collection of cable ties to his helmet after he was attacked four years ago. His injuries required medical attention.

"I was riding along Forest Road [Miranda] when one just landed on me and started ripping into my ear,'' he said.

"But I've been fine since, I think, thanks to the helmet, which has become a bit of a joke around the club.''

An Australia Post spokesman said numerous posties had experienced trouble with magpies this season.

"Before each season our delivery managers hold magpie briefings with posties to discuss why magpies swoop and the types of things they can do to try and avoid swoopings,'' he said.

"Some posties find that putting a flag on the back of their bike can distract the magpie, others draw eyes on the back of their helmet to try and trick them.''

A spokesman for Sutherland Shire Council said signs were installed in swooping trouble spots to warn cyclists and walkers.

"Sutherland Shire Council responds to between 20 and 30 calls from residents concerned about swooping birds during the breeding season.''

Where are the magpie swooping hotspots?