The National Eagle Center in Wabasha uses the usual choices of window decals to discourage bird strikes on the very large expanse of glass covering its south wall. We have some of those decals on our patio doors. They work, sort of. We bought something at the center during a recent visit that might do a better job, a product used by the center in addition to those decals. It’s a small container of fluid, much like a Magic Marker. The fluid rollls on, and once dry leaves an ultra-violet track, visible to birds but virtually invisible to you. It’s called UV Liquid, price $19.95 for enough to treat several windows through the year. It should be washed off and replaced every two months or so, we were told. It is visible until it dries, then all but disappears. We’re giving it a try. If it works, it’s clearly a good idea.