No, I’m talking about barn swallows, the topic of this thread.
They, and all other bird droppings, are acidic. They absolutely can cause structural damage.
I’m good with coexisting with lots of things. They’re welcome to live anywhere on my property except inside my structures. I don’t want their unsanitary poop on my hay, my tractors, my loft, my anything. It’s also a fire risk if they nest too near to wiring or lighting or fans or what have you.
Different strokes for different folks. They’re not welcome in my barn. (and I don’t have cats, either - similar reasons - they will use my stone dust floor as the worlds biggest litter box and I’m just not into it - not to mention the devastation they cause to natural bird populations).
Maybe I can put up a bat house for insect control.