When in choir dress, a Latin-rite cardinal wears scarlet garments — the blood-like red symbolizes a cardinal's willingness to die for his faith. Excluding the rochet — which is always white — the scarlet garments include the cassock, mozzetta, and biretta (over the usual scarlet zucchetto). The biretta of a cardinal is distinctive not merely for its scarlet color, but also for the fact that it does not have a pompon or tassel on the top as do the birettas of other prelates. Until the 1460s, it was customary for cardinals to wear a violet or blue cape unless granted the privilege of wearing red when acting on papal business. His normal-wear cassock is black but has scarlet piping and a scarlet fascia (sash). Occasionally, a cardinal wears a scarlet ferraiolo which is a cape worn over the shoulders, tied at the neck in a bow by narrow strips of cloth in the front, without any 'trim' or piping on it. It is because of the scarlet color of cardinals' vesture that the bird of the same name has become known as such.[citation needed]
Is there an answer to this question (If it cannot be answered, say "unanswerable"): The Cardinal bird was named as such because of what in relation to Cardinals of the church?
the scarlet color of cardinals' vesture