The Pelican-In-Piety refers to a very ancient symbol for Christ. The Pelican is known for the ultimate sacrifice to save its chicks. It prick its sides to feed them the blood that will sustain their life. Christ gave his blood for us, and at communion, the wine drunk is in remembrance of that sacrifice.
So the Pelican isn't just a bird - and what better way to make a child or adult understand the sacrificial nature of Christianity, than with a conversation begun about the bird on your vestment?