> they attract birds that that poo on vehicles

I think this is a tree density problem. Most cities have a small number of trees, and they’re almost always over cars. These are trees that line streets and parking lots. Without trees, birds just have telephone poles and wires, which are also over the cars.

In San Francisco, we have a lot of trees on most of our streets, and many parks small and big, all full of trees. This means birds spread themselves out everywhere, not just over cars.

I think the true barrier to getting more trees is that individuals tend not to want to pay for and maintain trees. This includes caring for the tree, trimming it when it gets bigger, and cleaning the pollen, leaves, fruits, and branches that fall.