I never knew live mealworms were calcium depleting!!! I do crumble “Peanut no melt suet dough” in with my mealworms now if I don’t have homemade. The bluebirds come at the same time every morning and most evenings show back up for a snack. I also have Blue Muffin Virburnum, Beauty Berries, Winterberries and Dogwoods in my yard. There are lots of cedar with berries on our farm. It is always a good idea to have natural resources in your yard.
After Christmas a local garden center has a workshop where you strip your live wreath of Christmas decorations and use it for an edible bird wreath workshop. The bluebirds, as well as all birds love it!!!
Carolyn, if you feel like putting something in the nest box for winter I would use clean pine needles or clean dried grassy material. Since this is what they use naturally. All of our boxes have nest cups and I never add anything but I live in a much warmer area than you.