You have been hearing a bird song for quite a while as you've slowly been approaching the marsh. Eack phrase sounds like a whistled question, "What is it? What is it? What is it?" repeated 3 or 4 times. Then the song itself is repeated. You try to pinpoint where the bird is, until you are sure that it's got to be just on the edge of that clump of cattails. Suddenly it jumps into view and you see that it has a greenish back. Then it turns slightly and you see that it is bright yellow in front and that it's wearing a black mask! What kind of bird is it?