You only see woodies certain times of the year because they are early migrators. Typically in my area, i do not see them after the second week ofthe season start,, oct 7-10 or somewhere in there.

I have about 20 woodduck boxes out, and 30 or so hen houses for mallards.. None of them are in trees,, all are placed on 2" fence pipe with predator gaurds on them. If possible you want the hole facing to the south. Do not use sawdust it can suffocate both the eggs and ducklings. change the woodchips yearly(get it at the petstore). We get about 75% use of both the boxes and mallard tubes,, with about 80% success on each structure used. Its a very easy thing to do, too improve the aeas you hunt.

Here is a good set of directions for the boxes.
http://www.ducks.org/media/Conservation/Conservation_Documents/_documents/duck_box_plans.pdf

Here is one for hen tubes.
http://www.deltawaterfowl.org/ddp/henhouses.php