You see, it's around this time of year that birds start to mate. (I've just conducted a brief field trip and didn't see any feathered fornication, but there was a frisky looking pigeon flapping about in a suggestive way.) How does this connect to Chaucer? Because he wrote a poem about birds falling in love. It's called the Parliament of Fowls, and if you like you can read it here.
Essentially, Chaucer dreams that he goes to the garden of love and witnesses a bunch of birds choosing their spouses.
For this was on Saint Valentines day,
When every fowl cometh there to choose his mate,
Of every kind that men thinke may
And that so huge a noise 'gan they make
That earth, and air, and tree, and every lake
So full was that unethe [hardly] was there space
For me to stand, so full was all the place.
And that is the first ever reference to Valentine's Day being the day on which creatures choose their sweethearts. Slowly the idea was transferred from birds to people, and that's the cause of all the trouble.
The Parliament of Fowls opens with some great lines on Love:
The life so short, the craft so long to learn,
Th'assay so hard, so sharp the conquering,
The dreadful joy alway that slit so yerne [slides away]
All this mean I by Love, that my feeling
Astonyeth [is astonished] with his wonderful working
So sore, iwis, that when I on him think
Nat wot I well wher [I don't know whether] I float or sink.
And then these bookish and lonely lines:
For all be that I know not Love in deed,
Nor wot how that he quitteth [pays] folk their hire,
Yet happeth me full oft in books read
Of his miracles and his cruel ire.
So unruffle your feathers, get your hackles up and find yourself a bird.