After reading the last chapter of xxxHolic, something hit me in relation to the symbols Yuko and Watanuki used to represent themselves, a butterfly and bird respectively. Back in the volume two, when Yuko had folded a butterfly to track someone, she mentioned that it could also become a bird if it had to travel farther. Meanwhile, in the completely different Clamp series Chobits from years back, Zima ends that series mentioning that children were meant to surpass their parents. Watanuki isn't meant to just be a cheap replacement- He may not be Yuuko's son, but he's her successor through and through, and one day he'll be more powerful than she ever was. Also, Clamp is really big on keeping track of their multiverse's rules. Zashiki-Warashi
Something hit me as I was reading this; Tanpopo. The little bird that Watanuki hatched and gave to Himawari.Regardless of your ship Watanuki gave something of himself, (i.e. his symbol) to her in the hopes of making her happy.
When the series gained the tagline "Rou" (meaning "cage") it was obvious that it referred to how the shop had become a cage that Watanuki was trapped in... but then he really became "the bird in a cage" when he decided the bird would be his symbol.