As a poultry owner who has had to retrieve some very fat hens from outside my fenced yard, I can assure you that while they do not soar, chickens do indeed fly. That, and your rather vague ideas about homing vs carrier pigeons and particularly their size (and never considering pigeons are also raised just for food), suggests a lack of familiarity with domestic fowl. Hence my two earlier suggestions that you consult an ornithologist before proceeding further.

As to building materials: the presence of square nails - or hand-cut nails, for that matter - is not a reliable indicator of age. Square (cut flooring) nails are still available, and before wire nails became widely available in the 1890s (note the late date) it was common to recycle old nails in new construction. Ditto "rough sawn" and "hand hewn" timbers, particularly in utility construction in rural areas, where I can attest it is still the case.

While you didn't come here looking for a discussion on pigeon size, that issue is germane to your initial query - which, I note, you began by observing UGRR sites in your neighborhood and describing your nesting boxes. The implication is that you're wondering whether those boxes constitute some sort of UGRR evidence for your house. But if those boxes aren't suitable for pigeons, then whether messenger birds were used in connection with the UGRR is immaterial, except as a point of general interest. Consideing the tone of your latest post, I doubt that out of the blue you began idly pondering the question.

Now you toss in the red herring that an UGRR documentary was filmed in your basement. That has no bearing on your nesting boxes, and being a movie set is not the same as being authenticated as an UGRR site. Had the latter been the case, I cannot imagine you would not have mentioned it.

In short, there is a strong indication here that you are making the common UGRR-related mistake of putting the appealing but unstable cart of wishful thinking before the slow, plodding horse of established fact. Were that not the case, rather than taking umbrage, you would take the time research what the boxes might actually be for, and THEN do your UGRR work. Unless, of course, you enjoy sending yourself on snipe hunts.