"I will not lapse into double entendres and ask how long this wonderful creature will 'stay up' in the square", the Mayor joked to the crowd, as he announced the arrival of the "big, blue... bird."
The fourth plinth remained empty between 1841 and 1999, and recently has been used to showcase modern sculptures.
It has been suggested that the reason for the long delay in choosing a permanent sculpture for the plinth is that it will be used for a commission of the Queen riding a horse. The speculation was backed up by former London Mayor Ken Livingstone, who earlier this year commented: "The understanding is that the fourth plinth is being reserved for Queen Elizabeth II."