Corinna has kindly supplied a link to a fascinating article about how magpies are perceived:

Magpies, wherever they live, haunt folklore. Sometimes they appear as a sinister omen, but equally often as a friend. In the UK, a lone magpie is considered especially ominous and it is commonplace to voice a respectful enquiry as to the health of its wife and children. Conversely in China and Korea magpies are seen as bringing good luck.

Click on the link to read the article in full: In defence of magpies

Wikipedia has the following entry about the nursery rhyme referred to in the article

One for Sorrow is a traditional children’s nursery rhyme about magpies. According to an old superstition, the number of magpies one sees determines if one will have bad luck or not.
There is considerable variation in the lyrics used. A common modern version follows:

One for sorrow,
Two for joy,
Three for a girl,
Four for a boy,
Five for silver,
Six for gold,
Seven for a secret,
Never to be told

Perhaps someone would like to write an article explaining how the Magpie was chosen as the symbol for the Society?