Our local organization, Prescott Bluebird Recovery, banded our nestlings for several years. I have mixed feelings about it. When the nestlings were removed from the nest and checked for parasites, etc., that may sometimes be of help although I never saw them find a problem with any of ours. On the other hand, I also have seen where the bands themselves may have contributed to the death of two nestlings when the banded legs got tangled in some kind of garden plastic material that was in the nest.
Knowing the identity is of great satisfaction in some instances for the person hosting the bluebirds. But I am not sure it is best for the birds.
Cari
Willamette Valley, Oregon