Mostly because the birds have different requirements. At it's simplest, the hole can be optimized that the bird will fit in the birdhouse but larger predators will not fit in. And the birds might have preferences on what kind of entrance they want. Like would you want a normal door to your house or a 5 meter high and 4 meters wide one?
Then the size of birdhouse, that also has practical reasons. If the birdhouse is too small, the eggs/baby birds can be too tight and can even be trampled to death. If the birdhouse is too large, the bird has to use extra energy to finish the birdhouse with moss and stuff, exhausting the bird for no proper reason.
And then the birds might have different requirements. Like the front wall needs to be rougher than normal so the chicklings can climb out. Or that the entrance wall needs to be minimum 5 centimeters thick so the bird accepts it.
A lot of reasons why there are different kinds of birdhouses :)
Do you take care of birds or work with animals? I really feel like you know a lot about it, and I don't remember animal caretaker being one of your many skills, do you think you can capture a photo of a bird in one of your birdhouses? That would be really nice.
No and no. I'm just trying to know why I'm doing the things I'm told to do :D It helps a lot to do things right when you know the reasons behind the requirements.
Also I've been thinking about building a birdhouse with a camera, but still thinking how I could do it the best way.
It would also be cool to see the different birdhouses you can make at some point and compare them to see how they look aesthetically. :P I don't know, I'll always be reading your content anyway XD