It was drizzling rain, in near-freezing temperatures, but the birders arrived before the sun rose, as soon as the gates to the park on the Maryland side of Great Falls had opened. With binoculars to their eyes, cameras around their necks, and masks on their faces, they peered into the brush and rocks around the Chesapeake & Ohio Canal National Historical Park, desperately searching for the elusive flash of blue, red and green.