Hi beer...They're celebrating an ancient tradition called hunting the wren who origins are lost in antiquity.

the tradition is referred to often in folk song, both in Ireland and England

Ditchfield in 'Old English Customs', p. 32, informs us that a wren-box was sold at Christies a few years ago which used to be carried in procession in some parts of Wales on St. Stephen's Day. It is about seven inches square, and has a glass window at one end. Into this box a wren was placed, and it was hoisted on two long poles and carried round the town by four strong men, who affected to find the burden heavy. Stopping at intervals they sang [The Cutty Wren]. And so on for eight more verses, taking the form of question and answer, as in the ballad of "Cock Robin", and describing the method of shooting the wren, cutting it up, and finally boiling it. (W.S. Walsh, Curiosities of Popular Customs)

See also The Wren (The King); Wren Song

[1962:] Some of the most ancient, most enduring and at the same time most mysterious English folk songs are those concerned with the attributes and sacrifice of monstrous animals. At the end of the 14th century, when peasant rebellion was in the air, the old magical song of the gigantically powerful bird (presented by a kind of folklore irony as a tiny wren) took on a tinge of new meaning. For here was the story of a great fowl so hard to seize, so difficult to dismember but so apt for sharing among the poor; and what did that suggest but a symbol of seignorial property?
(A. L. Lloyd, notes Ian Campbell Folk Group, 'Songs of Protest')