Great question. For a start, I'd like to point out that that's an obvious place for a bird to perch: On your shoulder. Your shoulder is one of the most natural and pragmatic places for a bird to sit. I'm also thinking about the two crows or ravens that sit on the god Odin's shoulders. His crows would fly off to look at the world and then fly back, sit on his shoulders, and caw all the news in his ears. So a bird on a shoulder might represent your connection to a larger perspective or an invitation to a birds-eye-view of things. It could be a reminder to think about the bigger picture, too. If we were birds ourselves, the shoulder would be the place where our wings started... so having a bird tattooed on a shoulder is like inviting flight or yearning for wings. Lastly, whenever I think about things on shoulders, I'm reminded of epaulettes, those regal shoulder decorations worn in ceremonies and celebrations.