I put up a pair of duck boxes on two beech trees about 70 feet apart, overlooking Prior Lake when we lived in Minnesota. In early March, we observed a lady wood duck standing on top of one while scrutinizing the other box. Back and forth she went a couple of times, finally settling on the left side box.
Other females joined her briefly in the same box, apparently donating the eggs they carried.
In mid April, we saw mom duck fly down and give several peeps, bringing a total of 17 baby ducklings flapping down to the top of the hillside. All were identical but some were hers and others likely adopted. Some landed on their heads but none seemed harmed in the process.
A couple more peeps and the duckies lined up and followed mom down to the waterfront.
Our best year/box in Bemidji was 18. Half woodies, half Goldeneyes.