Thought i would mention even at this late date the stunning spectacle of a flock of appx. 100 Tundra Swans in flight high over the Middletown/Burkittsville area.  Against a deep blue sky it was some sight!  

I need to add that my ID is best guess since i was without visual aids.  Peterson's guide (4th ed., 1980) comments: "often heard long before the ribbonlike flock can be spotted."  Certainly, i heard the calling, wondered briefly what it could be before thinking to look up.  Ribbonlike is the perfect description!  I believe swans is a safe bet.

Jim (Michael) Speicher
jugornought—at—yahoo-dot-com
Bethesda @work :(
Broad Run_S. FRED Co @home :)