While I appreciate the occasional check-in from a neighborhood bird on my windowsill, I don't appreciate incessant pecking at the window's glass. During mating season, male birds will seek to claim the best nesting spots in town, and your window sill might seem like a nice location. Under the right conditions, though, your window becomes reflective, and that male bird sees his reflection and thinks he's got competition. All that pecking is him attacking the “other” bird to defend his spot.
And it’s not just pecking, sadly—that reflection can cause birds fly into windows, too, which can cause much more damage (to the window and the bird). But as we enter mating season, there are a few things you can do to protect those birds and your windows.
Since birds can’t perceive glass well, they merely see what is reflected in it. If it reflects the sky and clouds, birds don’t realize they’re flying into a barrier. If they see their own reflection, they’ll think they’re seeing another bird. The key is to remove the reflection—and there are a variety of ways to do that. Keeping your shades, curtains, or shutters closed can help, but the glass may still reflect from some angles. In fact, if you don’t obscure the glass in some way, birds can see large plants just inside the window and perceive them as safe, outdoor plants to fly into.
Frosted or patterned glass has less of a problem, and you can purchase film to create a frosted look on your windows You can also use soap or tempera paint on the window to cut the glare—both are temporary and removable. Decals or mylar strips may help, but not as effectively as a solution that covers the whole window. In order to best deter birds, the solution is to cover the whole window in a grid that is small enough to deter even small birds from trying to “fly through.”
Acopian BirdSavers are a paracord grid you hang outside your window, and you can purchase them or make them yourself. This is the method used by the Cornell Lab of Ornithology.
Ultimately, if you have a pecking bird at your window and you want to protect the bird and stop the noise, the solution is to find a way to obscure the window from forming reflections.