Weaving in and out of the logpile, it could be a mouse. No, it's a wren. Now, if the wren is the king of all birds, as said in the rhyme, why does it have to be killed? And why is he the king at all? The answer to that is believed to go back as far as Aesop's fables. You know how the birds had a contest as to which could fly the highest? The eagle soared above the rest, but didn't know he was carrying a wren, who flew a yard or so above the eagle when it reached its top flight. His title wasn't contested by other birds. And in several languages the wren is named king. In France roitelet, in German Zaunkonig or hedge-king; in Dutch it is winter-king. But the eagle put a hex on the wren, so that it normally cannot fly higher than a bush.
And why must he die? In order, some say, to pay the debt to nature: nature feeds man and all creatures as long as they pay back the debt by returning their flesh to the earth. In certain hunting tribes, the hunter's body was exposed after death to be eaten by the wild animals. Back to the wren. In the 1940s the Irish Folklore Commission sent out a questionnaire to its correspondents on the Wren Boys tradition. From the hundred or so replies, most showed that the wren was killed on Christmas Day, but the ritual took place on St Stephen's Day. The dead bird (not all customs demanded that the bird be killed) was placed in the centre of a holly bush with paper flowers.
The wren boys were normally bachelors, and always disguised. They wore straw costumes or masks or animal furs. At least one wore womens' clothes. In Kerry they had a white mare, like a pantomime horse, covered with a sheet. There was music and the boys demanded money or food. At the end of the day, the wren was buried, sometimes in a coffin. (On the Isle of Man, burial was in a real cemetery.)
The food and money went to a wren dance; it was a courting occasion for the men were bachelors. Once again, the continuity of nature and life. These are a few pickings from an article in Bealoideas 1996-97. The particular contribution The Irish Wren Tales and Ritual, subtitled To pay or not to pay the debt of nature, is by Sylvie Muller, and space is needed to do her real justice. Her doctoral thesis, in Nice University on the wren customs, consists of 1908 pages and 144 plates. It is available, among other centres, at the Department of Irish Folklore at UCD. Fascinating.