In the Middle Ages, many people were poisoned by rye contaminated by a toxic fungus (ergot) which caused seizures, psychoses, and hallucinations (ergotism). The onset was heralded by pain and intense itching. The superstitious believed the only cure was a pilgramage to the shrine of St. Anthony, and another popular remedy was magpies baked in a pie. The pilgramage occasionally worked, because the afflicted left the source of the contaminant (the rye) behind. A French physician traced the cause to moldy rye in 1630, well in advance of the volume of verse from 1744.
Thanks for the article. I have only one small, nitpicky dispute.
“…the 24-letter alphabet only existed between the 10th and 11th Centuries. (W appeared in the 11th Century to condense UU, and J in the 15th Century as an initial form of I.)”
“W” did originate in the 11th century, but isn’t it true that although “u” was used as a medial form of “v” as early as the 10th century (and possibly earlier), it wasn’t considered a separate letter until roughly the same time “J” appeared? If so, it seems that the 24-letter alphabet (consisting of our modern letters minus “J” and “U”) would have existed – even if unoficially – for longer than Dex claims, and certainly during the time the rhyme was created. (In fact, “U” as an independent letter seems not to have become firmly established in English vntil the 17th century.)
(I don’t support the theory that the 24 blackbirds represented the 24-letter alphabet, but just want to show that it’s plausible even if not likely.)
Maybe a better scholar than I can rip my poor argument to shreds (or, to put it more politely, shed extra light on the matter). Actually, I’d welcome that chance to learn.
Of course, the plausibility of the “24-letter alphabet” theory (hereinafter “the alphabet theory”) depends on just how old the rhyme is. If the rhyme actually dates from a time when “U” and “J” were seen as independent letters, then the alphabet theory isn’t very plausible. However, the evidence suggests that the rhyme, like many folk rhymes, was known first through oral tradition and originated much earlier than its earliest printed appearance, making the alphabet theory plausible to those who have nothing better to do than see symbolism everywhere. 
FEotU said
Cite, please!
Rats, I can’t find my original source (it was an article I read), and also info from a documentary on TV (on TLC I think). I found some reference material on the net:
http://www.pearson-college.uwc.ca/pearson/fungi/ergot.htm
http://www.acsh.org/publications/priorities/0903/foodsafety.html
(the 2nd site was a page entitled “Food Safety Through the Ages”
Unfortunately, I can’t find a site mentioning eating magpies as a cure.
As a point of interest, ergotism has been suggested as the cause of the witchcraft trials of Salem, due to the hallucinations it can cause. If I remember a TLC program that tried to find a scientific basis for the 10 plagues of Egypt, the poisoned water and crop destruction by locusts resulted in a famine, forcing the Egyptians to give the majority of their meagre rations to their 1st born sons, causing them to die from the ergot poisoning.
In early Shakespeare, “u” and “v” are distinguished. However, just which was which seems to have been a choice left up to the individual (or indiuidval). I note, however, that in the First Folio, there are no capital W’s; VV is used instead.
Here’s my take:
Imagine if you will, a little girl among many little girls playing in a yard sometime before 1744. One of them says to another “Jane (we’ll call her jane) Sing us a Song of Sixpence!”
Since that phrase appears well over 100 years earlier, it must refer to something, and the something appears to be
‘a silly rhyme’ often made up on the spur of the moment and often beginning with the words “Sing a Song of Sixpence”. Here’s the format:
Sing a Song of Sixpence, de DAH de DAH de DUM,
de DAH de DAH de DAH de DAH,
de DAH de DAH de LUM (i.e. rhymes with DUM)
The sillier the better, the goal being to make the other kids laugh. The fixed meter probably aids in various games such as jump rope, etc.
Now, since the earliest reference for the poem refers to naughty boys, and since little girls often make fun of naughty boys, my guess is that this poem is based on this particular little’s girls notion of justice for all the naughty boys they knew. They’ll really be in trouble, because they will be baked in a pie (not really killed of course!!!) and set before the king (the most scary punishment imaginable to little girls and naughty boys, since the king is the ultimate authority. Whereas nowadays parents might say “I’m going to call the cops and have you taken to jail” to control their naughty boys, back then the threat of being taken before the king might have served the same purpose. I’d be surprised if parents didn’t use it.)
When the pie is cut open the boys will sing like birds (I don’t know whether it means that they will be forced to sing as punishment, or they will be crying in fear of the king and begging for forgiveness, and this is imagined by the girls to be singing like birds as a metaphor.)
the second half of the song is mostly filler, until the shocking ending that would send most little girls into paroxysms of giggles, and would also sear the poem into memory, and ultimately into history.
I present this hypothesis as total guesswork with no scholarship whatsoever to support it. To me it just seems the simplest explanation.
The transformation of naughty boys into blackbirds might have come later when the ‘birds’ reference got muddled with the later solitary blackbird, by people less motivated by punishing little boys. Perhaps it was changed by the little boys themselves 
!