I prefer to be skeptical of claims of deep antiquity for this custom. In antiquity it was considered unlucky and cowardly to kill the wren. The wrenning custom might be an ancient example of ritual topsy-turvyness. Or it might be much more recent, dating from a time that the old taboo had weakened. Or it might have no connection with the old prohibition at all. Killing or capturing a bird was simply one possible sport to engage in in a festival day, the wren being chosen because it was one of the birds that didn't migrate; taking it house-to-house was simply one possible way of getting money or food.

T.