When that person approached trees in which the birds were nesting, the magpies called out and pursued him.
The findings are published in the journal Animal Cognition.
One of the researchers, Sang-im Lee from Seoul National University, explained that the findings were "accidental observations".
His colleague, Wonyoung Lee, climbed up to the birds' nests during the breeding season of 2009.
During a routine nest-monitoring study, "[Wonyoung] would count their eggs, measure the chicks, and take blood samples from them," explained Dr Lee.
But when Wonyoung later passed close to a nest that he had previously climbed up to, the adult magpies behaved very aggressively towards him, following him and making loud alarm calls. Read the full report here.
Source: by Victoria Gill, Science reporter, BBC Nature News.