A project to transform a birder’s paradise into a children’s playground and garden in Prospect Park could help lead to the local “extinction” of already-suffering migratory fowl, critics say. The $37.5 million plan to revitalize the dilapidated and overgrown Vale in the northeast corner of the park would take away precious shelter for millions of warblers, sparrows and thrushes during their biannual voyages — simply to indulge humans, birders argue. “When you see the habitat disappearing slowly