Midnight Migration PuzzleYou may have seen geese migrating in a V shape overhead, honking loudly as they go. But what about songbirds like warblers, grosbeaks, and thrushes? They migrate under cover of darkness, when conditions can be more favorable for flying, and safer from predators. Though you may not see them, if you listen carefully, you may occasionally hear short, high pitched call notes coming from the night sky. Warblers that weigh as little as two or three nickels