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.
Birds are most likely to use a nestbox that approximates natural cavity choices.
There are many nestbox styles, each of which have their own pros and cons. No matter what species you are trying to attract, you should choose or build a box that is sturdy, has the right size entrance, will stay dry inside, opens for cleaning and monitoring. There are also many choices of material (wood, PVC, other) and a variety of types of predator protection.
per Bergmann's rule, some species of birds may be larger in colder climates - e.g., a Canadian starling may be bigger than a Florida starling, thus the Canadian bird might not be able to fit in a certain hole size that its southern version could squeeze into.
Many people find that boxes with a 1.25" or 1 1/8" hole restrictor on them before an egg is laid are not utilized, even by smaller birds like titmice and chickadees.
If the entrance hole is enlarged (e.g., by woodpeckers or chewing rodents), put a hole reducer on it.
Here are some commonly accepted birdhouse specifications (some sources disagree, so there may be multiple listings. Also, I do not know how reliable this information is - sometimes misinformation gets propagated).
*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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