Agree with Woodster's comments above. I have 10 houses and the Woodies definitely utilize them, along with the occasional owl.
The houses require appropriate design, positioning and upkeep and there are lots of sources on the web that provide detailed information along those lines. I see definite increases in the Wood duck population as a result of placeing the houses on my lake and ponds. Have fun with it!
Here are two sources for Wood duck houses and pole guards that I found useful.

http://www.drsfostersmith.com/product/pr...CFQ6EaQodCosGDg

http://woodducknestboxes.com/duck.html