User blog comment:Hawkinz340/What is Bird Cage?/@comment-24638955-20140410161049

If my memory is right Dresrosa is in a rock island and Pica can control/manipulate rocks, the thing  is he would be making the island take the form of a birdcage and maybe reinforced with Doffy strings so anyone can't enter or escape, at least easily. This should help Doffy continue one of his plans that requires the set up of the birdcage.