Mary Poppins/WMG

Bert (at least in the movie) is as magical as Mary.
As a jack-of-all-trades, he's clearly a master of at least a few; and he has a unique sense of perspective. He and Mary have "history", and he always seems to be in just the right place to help out or liven things up. He's always in just the right job (even if the kids don't know it yet) at just the right time. Good-natured happy-go-lucky drifter or Chessmaster of the entire MP cosmos? You be the judge.
 * This could be chalked up to his having drawn it, but in "Jolly Holiday," he does have God-like control of the painting world.
 * This is made explicit in the stage version. Bert spells out the words "Welcome Back" in the clouds right before Mary returns at the start of Act Two and later pulls a real bouquet out of a painting of one for her.
 * Bert was a child Mary helped raised. Growing up, Bert fell in love with her and knows enough about her to even "sense" she was coming at the beginning of the movie.
 * Bert having magical ability would explain why he and the other chimney sweepers can fit down tiny chimneys and bat fireworks around without their brooms sparking.
 * Bert is Santa Claus?
 * Inspired by the play: Bert is a guardian angel tasked with watching over the section/streets of London the Banks live in. The other chimney sweepers are guardian angels from other areas of London. It explains his status as The Narrator - it's because he's omniscient.
 * Bert is as magical as Mary Poppins because he is her opposite. She is a force for order and he is a force for (non-malevolent) chaos.
 * I've always thought this to be the case. Thanks for providing many interesting and equally plausible variants!

One of Bert's many jobs is posing as Mr. Dawes, Sr.
Bert adopts many jobs out of narrative convenience; but throughout the film, his main duty is to demonstrate the folksy wisdom of the underclass. Perhaps he also puts on a bald wig and a fake beard in order to demonstrate the corruption of the upper class. Plus, have you ever seen Dawes Sr. and Bert in the same room together? Think about it.
 * This doesn't explain how Mr. Dawes Sr. has a son, or his death at the end of the film.
 * It's like Peter Pan, where the same actor plays Captain Hook and Mr. Darling. The main male characters, George Banks, Bert, and Mr. Dawes Sr., all embody the notion of loving your work. Bert and Dawes are the flip sides of that idea: Bert loves his work, but he always takes time to have fun, even on the job. Mr. Dawes Sr. loves his work too much, to the point that he's utterly clueless of what fun is. Banks lies in between them. He starts out with little thoughts for fun, but is in danger of becoming like Dawes, where he thinks only about his work.

Bert is a member of the Dawes family.
He might even be Mr Dawes, Jr.'s son, making him the grandson of Mr. Dawes, Sr. There is a family resemblance, though obscured by Dawes, Sr.'s advanced old age. Always more of a free spirit than the rest of his family, making him quite the black sheep being from a long line of bankers, he ran out and was summarily cut off from his family fortune. This was quite all right by him, as he obviously prefers getting by doing exactly what he wants. He takes great pains to distance himself from the rest of his clan, going so far as to affect a terrible Cockney accent so as to sound nothing like them.

The woman who feeds the pigeons is Mary's mentally ill mother.
Very few people know about her, and Mary looks awfully wistful as she sings about her. Why would Mary Poppins, who has spent nearly all of her time as a magical nanny, know about this obscure person?

She knows of her because they are related.
 * Mary uses her universe-bending powers to help children. Mary's mother sits feeding pigeons all day. This may be because Mary's mother has forgotten about her powers, or because she became so disillusioned with humanity that she decided to tend to birds instead.
 * Or she has no powers, she became disillusioned when Mary left her after somehow becoming magical to help kids all over the world.
 * Or she traded whatever powers she may have had, as well as reducing her life to that of a poor birdseed-seller, in order to secure incredible magical power for her daughter. Mary is indebted to her, but cannot help her because of the contract.
 * Or on the other hand; the old woman was a child that Mary helped many years ago.
 * Or, in that case, failed at helping
 * She can't help her because Mary is the birdseed lady. She can't help her becauase you can't meet yourself when you time travel.
 * Alternatively, the birdseed lady is more enlightened than Mary. The birdseed lady helps everyone, even the lowly pigeon, and she encourages others to do so. She could easily help children, but she understands that you can only help a few children at a time. By sitting outside the cathedral, she can reach out to everyone who passes by. Whether someone actually buys a bag of feed or just smiles approvingly, she has done her job.

The Bird Woman is an older version (a much, much older version) of Mary.
It's implied that she can communicate with the pigeons, or at least believes she can. Mary can communicate with them as well.

Mary-from-the-future came back to make present-day Mary's lesson to Michael more effective.


 * This seems very logical to me. The fact she vanished from the cathedral steps would be because her lesson had been taught to the children (and, if she did set off the run on the bank, had made her point to Mr. Banks) so she was no longer needed and went back to the future. The fact this made it seem she had died so as to emotionally affect Mr. Banks was just the icing on the cake.

The Bird Woman is a Saint.
She's the Earthly manifestation of a charity and compassion. When the children refused to feed the birds, she struck back by causing a run on the bank. She's not malicious, just somewhat playful with an odd sense of humor. Mary's line about the "Saints and Apostles" watching was literal. If you show that you care, they smile upon you. If not, then you're in trouble.

The Bird Woman died.
She's a homeless person on the streets of London; it's highly unlikely that she would have survived for much longer. I know Mary says that the Bird Woman appears "early each day", but there's just something about Mr. Banks's body language as he looks at the empty steps of the cathedral.
 * First, the absence of the Bird Woman is enough to give him pause, then enough to divert his attention entirely. He's taken this route many times before, so he's familiar with every step of the route. Now, in the dead of night, there's nothing to distract him from the imminent meeting he's about to have with the board of the directors. It takes this particular absence to distract him.
 * As the camera switches to the empty steps, the background music swells to the same level as it was in Mary's song earlier, giving this particular moment more poignancy.
 * There's no Bird Woman, and no birds either. Obviously, after her death, they had no reason to stay there.
 * Banks stops and looks up into the air - is he looking at the cathedral itself? Or is he looking for the absent birds?
 * What seals this for me is Banks's expressions. Up until now, his emotions almost project a sense of annoyance at this absence in his walk; as he brings his head down from looking skyward, however, he very visibly pauses, and this is the point where he starts to become forlorn. He looks again at the former seat of the Bird Woman, and has to hesitate before turning away. Tomlinson acts this scene superbly well, and it seems clear to me that Banks has realized that the Bird Woman has died. This plays well into the film's moral of "Always cherish your time with your children, because you never know when they - or you - will be gone."
 * This is not so in the stage musical; it clearly shows the Bird Woman post the bank meeting.
 * Very plausible and poignant to me; great deduction and analysis!

Mary Poppins is a God trapped in a human body.
No amount of witchcraft could produce an entire universe made from a chalk drawing. Clearly, Mary is a deity.
 * So I take it Harold from Harold and The Purple Crayon, Rudy from ChalkZone, Paint Roller from Kirby, Michael from Little People, and other characters listed at Art Initiates Life are gods too.
 * In the book, it was implied she was of The Fair Folk, though that was an uncomfirmed theory the children had about her.

Mary Poppins did not exist before the events of the movie, and was created Hogfather-style, by the children.
Probably every child who has ever had a nanny wishes for a "practically perfect" nanny. In the Discworld book Hogfather, it is shown that if there is enough free occult space, anthropomorphic personifications can be created by someone stating that they exist and others agreeing that it makes sense. Jane and Michael's song perfectly described Mary, and, combine with thousands of children's worth of belief, created her.

Mary is immortal.
Because she's a witch.

Mary Poppins has a different guy in every town
Her job is very transient, and so it may be that Bert is not the only man in her life.

The entirety of Mary Poppin's visit is dreamed up by the children
Mary Poppins is simply a normal (but better that usual) nanny who somehow managed to catch the children's interest. They dreamed up some fantastic adventures and thought up all these things.
 * Well, that could be one possible explanation for her denial when the children tried telling the adults about their chalk drawing adventure. She remembers no such thing as it was all just their imaginations.

Mary Poppins is the mother of Madame Pomfrey.
They're both magical British caregivers.

Mary Poppins and Nanny McPhee are sisters
They're both magical and fairly no-nonsense. They both started their nanny careers together, mysteriously showing up out of nowhere to help families with rambunctious children and parents who have troubles communicating with them, and then leaving as soon as everything is fixed. However, due to differences in opinions about what sorts of disciplinary actions should be taken, they went their separate ways.

Mary Poppins is the mother of Maria Von Trapp.
Think about it, Mary was a young woman during the Edwardian Era, which means she would probably have been in her thirties during the end of WWI, when Maria must have been born if we assume she was in her twenties when she married Captain Von Trapp in 1938. They both share a similar apperance, a similar singing voice, and a similar gift for being the nanny for children that have previously scared away several predecessors.

In the books, Mary Poppins is Nyarlathotep
The movie depicts her as kind and loving, but in the books...

Mary Poppins is on the same wavelength as Willy Wonka, and possibly his mother.
Mysterious benefactors of lonely children, both seem to exist to teach both parents and children the benefits of etiquette, temperance and honest self-expression. Furthermore, both seem more than a little hung up on the spiritual applications of sugar, and both seem to have the power to transform foodstuffs into various colors and flavors at will. Mary Poppins' love of travel and wide circle of acquaintances made Wonka come to distrust the outside world, withdrawing to his magical factory. I would not be surprised if Bert's last name is "Bucket".

Mary Poppins worked for Hogwarts
She went around Wizarding England finding muggleborn magicks and accustoming them and their parents to the wonderful weirdness of the wizarding world. She trains them not to speak of their adventures to anybody else, hence her denial post chalk drawings. The winds changing is just her getting an owl off screen telling her there are some more children who need her service. She was probably in Ravenclaw when she went to Hogwarts and excelled so well in Muggle Studies and muggle interactions she was offered a full time job.

Bert is a squib, in touch with the magical world and an old friend of Mary Poppins, but has no talent himself. All his tricks come from Magical objects, such as the chalk and the sweepers. Uncle Alberts laughing disease is a real wizarding sickness, Mary wasn't kidding when she said it was contagious. It's caught the same way as the common cold and the symptoms activated by laughter. Micheal passed it on to Mr. Dawes when the shook hands, but it took a while for the symptoms to manifest themselves as he had no sense of humor.

Sadly, Mary Poppins is never mentioned nor seen in Harry Potter canon because she was killed during the first rise of Voldemort for her unacceptable fondness for muggles and muggleborns. The children whose lives she touched passed on her story untill it got to Walt Disney and her abilities greatly exaggerated over time and for appeal for the film, so the children actually go into the pictures instead of watch them move around and Mary sits on clouds. You can go cry now.


 * ...damn, that is now my canon.
 * Expanding on this, Mr. Banks is like Aunt Petunia. That is, he was jealous of a sibling who had magical ability while he didn't, so he grew up to become obsessed with tradition and object to anything out of the ordinary. Note when he hears about Mary Poppins' outings, he doesn't seem to consider them impossible, he just objects to them because they are not "proper".
 * Wow. Both beautiful and sad at the same time. *sniffs*

Mary Poppins is a Time Lady, and her carpetbag is her TARDIS.
Oh, come on! How come no one else put this one up already?!
 * We did.
 * Bert was her companion at some point
 * The umbrella is a sonic screwdriver in disguise. Alternately, there's a hidden blade.
 * Jane and/or Michael will grow up to harness the power of laughter to use antigravity, a discovery vital to mankind that would neve have happened if Mary Poppins hadn't taken them to Uncle Albert.
 * It's Michael, and not only would he harness it to create antigravity, but he would also be employed by Sony, where he would eventually combine the upward force of laughter with the downward force of sadness to create the Emotion Engine.

Mary Poppins is related to Elphaba
Through the movie, a lot of screen time is devoted to Mary Poppin's feet. Fetish Feul or not, her feet are always pointed in opposite directions...just like the Wicked Witch of the East.

Mary Poppins is an alcoholic
Rum punch anyone? Mary never takes the children on any adventures unless you count the ones down to the local gin shop or liquour palace. The trips into the street drawing and tea on the ceiling with Uncle Albert are booze induced fantasies. The time she spends surrounded by dancing chimney-sweeps is symbolic for what is really going on (something that can't be shown in a children's film).

Bert has a speech impediment
His condition is similar to foreign accent syndrome, but is milder, and was caused by moderate parental abuse rather than head trauma. He tells us all about this in "Supercalifragilisticexpialidocious":

"Because I was afraid to speak when I was just a lad My father gave me nose a tweak and told me I was bad"

This is why his Cockney accent is (seemingly) so awful.

Bert and Mary Poppins are siblings
They are clearly extremely fond of one another, but not in a particularly romantic way; instead they are siblings.
 * Bert is a few years older than Mary Poppins, and throughout the movie he enjoys teasing her and otherwise ruffling her feathers.
 * They also both refer to Uncle Albert as 'Uncle Albert' and not just Albert.
 * Relating to the above WMG, siblings would explain Bert being as magical as Mary, and they could easily be the children of Mr Dawes, Jr as well as the bird woman, who might've been left by her husband when she began showing signs of mental illness. Additionally, Mr Dawes was probably a father very much in the style of George Banks, who disinherited his free-spirited son, Bert, but continued to raise his younger daughter Mary into a proper Edwardian lady. Bert, however, remained in the area to remain a good brother to the sister he loved very much.
 * Finally, when during 'Supercalifragilisticexpialidocious' Bert mentions he learned the word that saved his nose from his nose-tweaking father--he learned it from Mary! (She may have even invented it just to help Bert when he struggled with speech.)

Mary, Bert, Nanny McPhee, and others are a part of a race whose purpose is to help children (and others while they're at it) learn something.
There aren't many rules. The members can work how they want to and leave how they want to. The only rule is that they must use their magic to help the children. Bert still hasn't gained much skill at magic, which is why we haven't seen him use it much. So Mary's boss (whoever it is) told her to become more or less Bert's teacher.

Mary, Bert, and possibly other characters are Anthropomorphic Personifications.
Mary would be a personification of Proper Raising of Children; Bert of Taking Joy in Your Work; the Bird Woman of Kindness to Animals. (Mary reveres her because Kindness to Animals is a much bigger thing to be an Anthopomorphic Personification of). Uncle Albert is another one, though I'm not sure what.
 * That's easy. Uncle Albert would be a personification of laughter and humor. The group has tea on the ceiling when they all start telling jokes and laughing, but come right back down when they get sad that they have to leave. Mr. Dawes, Sr. later starts floating around as well when he finally gets the "wooden leg named Smith" joke and starts laughing.