There has been a lot of legend associated with St. Valentine's Day. Even the precise saint being honored is not be known with any surety. However, by the Middle Ages, the date of February 14th was being celebrated as St. Valentine's Day.
One of the earliest documented references to St. Valentine's Day was regarding the pairing of birds in Geoffrey Chaucer's Parliament of Fowls. In it he states:
For this was on seynt Valentynes day,
Whan every foul cometh ther to chese his make
which, when translated to modern English, yields:
For this was on Saint Valentine's day,
When every bird comes there to choose his mate.
Due to the early date of the narrative, and the fact that no earlier record of this legend has been discovered, Chaucer may have invented the belief that birds began to pair up on St. Valentine's Day, but passed it off as historical thought. Rather the story is true or not, it was a somewhat common belief during the Middle Ages that St. Valentine's Day was the day in which birds began their courtship rituals and began to pair up and produce offspring.
Because of this history (or pseudo-history), birds have been frequently associated with St. Valentine's Day. Likewise, birds also appear on many stamps of St. Valentine's Day.
While not a St. Valentine's Day issue, Denmark produced a beautiful souvenir sheet consisting of two bird stamps in February 1999. The two stamps, depicted here, are stylized images created by artist Sonia Brandes using a technique called psaligraphy, or "the art of cutting".
The souvenir sheet can be easily found through your favorite stamp auction site for a small price. It will be a colorful addition to your stamp collection.