A ranger with Parks, Conservation and Lands, Stuart Jeffress, said magpies swooped to defend their nests while newly hatched fledglings were at their most vulnerable.

''If you encounter a swooping magpie, be sure to walk quickly away, not run,'' he advised.

Associate Professor Darryl Jones, from Griffith University, said male magpie parents had increased levels of testosterone. Although testosterone was usually associated with aggressive behaviour, research had found stress hormones related to the magpie's environment were predominantly responsible for attacks.

''Places like schools and highways are more likely to have swooping magpies in them because of the noise and activity,'' he said.

He emphasised magpie behaviour was ''agonistic'' rather than ''aggressive'', meaning the tendency to swoop is strictly a defence mechanism designed to protect young.

''The absolute majority of magpies don't attack humans the ones that do have had something happen to them in the past. Most are completely acclimatised to our presence; they don't notice us at all,'' he said.

For more, pick up a copy of today's Canberra Times