The holes in this Saguaro are from Gila woodpeckers and gilded flickers, who carve out nesting sites in saguaros. After the original birds move out, others, such as owls and sparrows, use the abandoned nests the next year. The cactus is not hurt by the holes, unless a bacterial infection sets in. This is fairly rare, as the cactus has a mechanism in place to quickly callus inside the hollow.
In one town I visited, a shopkeeper couldn’t understand why I wanted to know about such trivial stuff, and said no one he knew really cared about that kind of thing, that there were far more important things to think about, that I was the only person who had ever asked for that information, and that maybe I should just go buy a book. I left his shop empty handed, both in knowledge and goods.
I did run into a taxi driver once who was a genuine storehouse of data regarding the area I was visiting. He answered every question in detail, volunteered fascinating facts about the flora and fauna of the area, and did it with the aplomb of a tour guide describing his route. He received a very large tip.