Do you remember Chip 'n Dale? They were created by Walt Disney in 1943. It seemed wholly natural to viewers that they had a nice home inside the trunk of hollow trees. The idea that they were looking for natural cavities appeared from their very first story, which had to do with their attempt to settle inside the barrel of a cannon.
Yet, hollow trees have a special fascination and are part of forest lore everywhere in the world. They are not just fascinating for human beings, they are also home to all sorts of animal species. Birds, typically, but also larger ones, such as raccoons and even bears. In this sense, hollow trees are a feature of the forest holobiont, just one of the many multispecies holobionts that keep the ecosystem alive and adaptable.
The formation of a hole on a tree stem is a wholly natural process that's generated, normally, by the action of specialized saprophyte fungi -- although woodpeckers can initiate the process and, sometimes, dig quite substantial holes. The plant is not normally harmed by one or a few hollows. Old trees tend to accumulate hollows, and when they die they become "snags," not anymore live trees, but still part of the forest ecosystem, homes to a variety of animal species.
So, why so few hollow trees around? Have they become an abomination? Apparently, yes, I have a fig tree in my garden with a few hollows in the stem, and everyone who sees it asks when we are going to cut it. And that's the destiny of hollow trees everywhere. If you look at the term on the Web, you'll find plenty of pages describing "hollow tree removal services." It seems that hollow trees are indeed seen as a monstrosity, slated to be eliminated as soon as possible. And whenever a tree develops a hollow, it is plugged with cement or silicon, or whatever.
More in general, hollow trees are a victim of the "optimization" trend in forest management. It doesn't matter whether it is a plantation or a park, if the idea is to make trees grow as fast as possible, then hollow ones have to be removed. It is typical of human management: it is aimed at maximizing just one of the parameters of the system. Instead, holobionts aim at optimizing all the parameters together. Humans aim at yield, holobionts aim at stability. Maybe, one day, humans will learn, and maybe they are already starting to learn. In the book "Chanterelle Dreams" by Greg Marley, you can find an entire chapter dedicate to how to restore tree hollows for cavity nesters. Some creative methods can be used, such as using shotguns to shoot slugs packed with fungal spawn into the bulk of a tree stem. It seems to be working -- so, not everyone marching into a forest with a shotgun in hand is there to do damage!
Up to not long ago, I was convinced that, apart from my fig tree, hollow trees had been scientifically exterminated in all the areas close to where I live. Instead, I discovered that the avenue just near my home is lined by wonderful honeyberry trees ("bagolari" in Italian), most of which have numerous, well-visible hollows. Some are clearly the result of branches having been cut off, others may have developed by themselves. I won't tell you where exactly this place is, least someone decides to cut these trees for being "dangerous" or to plug the hollows with cement. As far as I can say, these cavities are not inhabited by birds or other animals, probably because they line a trafficked road. But it is nice to know that there are places where owls, squirrels, and maybe Chip and Dale could take refuge if they decided to live in this area. Here are a few pictures.
