@johnnye: I can see two potential tropes here:
1. Thieving Magpie: Magpies, rooks, and by extension other corvids have a reputation for Sticky Fingers and a love of shiny things. This would be a subtrope of Animal Stereotypes.
2. One For Sorrow, Two For Joy: Magpies are a source of superstition, with various rituals and rhymes being associated with this.
There are also several examples of "some magpies show up", which should be cut.
^After reading all the replies here and re-reading the trope, I’m coming around to most of what you’re saying. I absolutely agree with your potential "Thieving Magpie" trope and your statement about “some magpies show up” (and those definitely should be cut).
I also agree that there’s a second possible trope and like your later renaming suggestion of "Counting Crows." However, I think this second trope needs a new description: for starters (and I have no problem if this undergoes a lot of editing), I would go with something like, “…the act of predicting (or the ability to predict) the future based on how many magpies, or other corvids such as crows, you count. This trope is based on various versions of a British nursery rhyme on this theme; typically in these, one magpie is associated with bad luck or sorrow.”
In addition, perhaps as crazysamaritan suggests, even for a renamed trope, “…the first seven lines of the poem should be used as the page quote to explain the name,” although some versions will be better than others, and we should give the choice of version some thought.
edited 2nd Jan '14 7:04:57 PM by SciFiMs