A 1 1/8 reducer will keep a HOSP out of the nestbox. They need a 1 1/4 inch to get in. I have two nest of Dees right now, each incubating 6 eggs (I had 8 & 9 count last year). One of my boxes is just in the woods line, that one has a wren guard (with sides) on it as we always get wrens. Wrens have been here for well over a week now, maybe two and so far so good. My other box with Dees is not in wren territory. During nestbuilding a HOSP was trying to claim the box. I put up a sparrow spooker and she went on to lay the 6 eggs. The other day a pair of Bluebirds were sitting on the sparrow spooker so I added a 1 1/8″ reducer the to box. The Dee accepted it right away. Hopefully they will make it. Chickadees are very tolerant of wren guards even put up during nestbuilding. Just don’t add the sides right away. If no bird can see the hole, they are less likely to try to check it out.
Sorry about your Dees, so sad!
Donna in WI