Of Harriet Tubman and great blue herons – simply retired Maybe not everyone thinks of pairing bird-watching with African-American history. Well, we do. And near Cambridge, Maryland, you can do just that. We recently spent a spring day visiting the Harriet Tubman Underground Railroad National Historical Park in rural Dorchester County, Md., followed by a drive through Blackwater National Wildlife Refuge. The trip was about 60 miles from our home in Sussex County, Delaware, on two-lane roads past farm fields,…