i worked with a prof last spring on a continuing wood duck nest box project the maintaince that we did was check to make sure that the lids were still there and the wires that held the lids down (predator proofing if you will) were not rusted and brittle, we also changed out some of the nastier rotten sawdust that is in the bottom of the boxes and remove a beehive and wasp nest, getting at those two boxes sucked and were ultimately replaced after a healthy dose of beebop. out of a total 18 boxes that the JC here has i had to replace 6 complete boxes, counting the bee and wasp boxes, due to weathering and predation from 2 legged pests.
last spring we had a total of 4 active nests with hens sitting on eggs and one egg dump box, was not an overly great box use percentage but its 4 more nests than there would have been without the nest boxes.