Recent research that some birds can recognise  individual humans has been further confirmed by simple experiments on Magpies in  South Korea.

Scientists from Seoul National University were  performing an annual breeding survey of Magpies around the college campus, when  they noticed that the birds seemed to mob and scold only the two members of the  team who were accessing the nests.

One in particular, who was counting eggs,  measuring chicks and taking blood samples, seemed to elicit an aggressive  mobbing response, almost as if the birds remembered him personally. Intrigued by  the observation, the team performed a series of experiments to test the idea  that the birds really did recognise Woun Young Lee, the ‘climber’ and main  author of the published paper. Tests were set up to record the reaction of
nesting Magpies to climbers and non-climbers and to pairs consisting of  both.

Of the 11 Magpie pairs tested, significantly more  than half of the territorial Magpies showed no response to either a climber or  passerby until their nest had actually been visited by one of the team. During a  nest visit, the Magpie pair would scold the climber, even attempting to  physically attack him by pecking at his head, and sometimes even being joined in  the attack by neighbouring Magpie pairs.

Thereafter, the birds would mob the climber, but  continue to ignore random passers-by. When a pair of people consisting of a  climber and a non-climber approached the nest, mobbing would begin, but the  birds would only pursue the climber when the two humans separated. At most,  birds would fly away when approached by a human of whom they had no previous  contact, with no active aggressive response seen.

clearly, Magpies in this particular urban environment are able to distinguish between individual humans, possibly to isolate a potential threat among thousands of similar humans in the overcrowded city. Magpies are known to quickly habituate to non-threatening models of owls, and non-threatening humans appear to be similarly ignored.

The authors speculate that the birds use facial  features to differentiate between human individuals, as both climbers and
non-climbers wore random selections of clothing in the experiments, hats were  swapped around, and other human pairs wore identical clothing. all participants  were Koreans with comparable facial features and black hair, as well as similar  builds and heights.

In the United States, similar behaviour has been observed in American Crows and Northern mockingbirds, and the crows were shown
to recognise facial features. previous workers have noted that Magpies were unable to recognise a previously-scolded individual when she wore a bandana across her face. More work is intended to be performed using face masks and comparisons between the behaviour of urban and rural Magpie populations.

Reference
Lee, W Y, Lee, S-I, Choe, J C and Jablonski, P G. 2011. Wild birds recognize individual humans: experiments on magpies Pica pica. Animal Cognition DOI: 10-1007/S1007-011-0415-4