One excellent way to help native cavity-nesting birds survive and thrive is to put up a nestbox(es). If are going to make or purchase a nestbox, you can select one specifically designed for the species of bird you want to attract. Different size birds with different habits have different preferences when it comes to entrance holes, interior volume, dimensions, mounting, etc. Note that many bluebirders find that boxes with 1.25" or 1 1/8" hole restrictors on them before an egg is laid are not utilized even by smaller birds like titmice and chickadees.
Here are some commonly accepted birdhouse specifications (some sources disagree, so there may be multiple listings. Also, I do not know how reliable the information is - sometimes misinformation gets propagated). I expect birds are most likely to use a man-made nestbox that approximates natural cavity choices. Note that there are many nestbox styles, each of which have their own pros and cons. There are also many choices of material (wood, PVC, other) and types of predator protection.
*Mountain Bluebird and Western Bluebird ranges overlap in some areas, and Mountain's need a 1 9/16" hole. Some Western's are larger than others. So the 1 9/16" hole is a safe bet, and will also exclude almost all starlings.
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