For folks at home thinking about their windows: Apparently one issue is that windows may mirror the outside environment to birds, appearing as portals to more open-space and trees, especially if you include ultraviolet light which humans can't see and which the glass wasn't designed to pass-through.

So there are a variety of products advertised for home usage that stick to the outside of the window to make it appear more like a barrier, often semi-transparent to us but more-opaque in ultraviolet to birds.