The suggestion is often knocked around that the original king of the birds for the purpose of this hunting tradition is the firecrest or goldcrest wren, because these birds have a kingly golden crown. The extension to common wrens, in this theory, comes later as the precise origins of the tradition become blurred in the "folk mind", influenced no doubt by the fact that it's a bloody sight easier to find a common wren. I believe customs in the balkans add wait to this theory, but I can't cite a source for this. I always add the same caveat asMalcolm Douglas: basically, I'm not a historian, I just read a lot of stuff and don't always make notes.