Bski
I am thinking of buying some infrared game cameras to set up around the ponds. Have you considered doing something like this? I don't think they are cost prohibitive.
PS: I have two confirmed sightings of Wood Ducks on my big pond. I can't relate how excited I was to see them - and I don't even have my boxes up yet. Easily my greatest surprise of pond owning in 09. Suh-Weet!