Alot of good info here from all that posted .I have put out several over the years Mostly had gray squirrels using them And sometimes owls also Once again all the cruel trappers that just want to kill everything going against that stupidity and nonsense and actually doing something to improved wildlife habitat I had some years ago built and put out a large number of bluebird boxes And it was incredible how many of them where being used by all types of birds and small mammals would rear an unreal number of young This is a good post and it can be an example to use to show kids how to get involved in wildlife There was a young boy that earned his Eagle Scout badge by coordinating a day of building different types of boxes And the boy that earned his scouting badge had contacted local land owners to find spots to place the boxes If there are county parks or public walking areas these boxes. could be a great way to educate the public about wildlife . So from locating the patterns the making the boxes and arranging for donations to purchase the materials and than setting up a few work days to get boxes built This is where the younger kids can be very useful to the process and than going out to the public or private area and hanging the boxes is a project that is valuable to the scouts too and the public And the biggest winner is the wildlife using them them

No expert, but those look like Robin eggs.

how long do you make them?