Sticking one or two decals on a window will not help. You might try one of these techniques, ranked roughly from most to least effective: Cover the glass on the outside with window screening or netting at least 2–3 inches from the glass, taut enough to bounce birds off before they can hit the glass.

Netting. Cover the glass on the outside with netting at least 3 inches from the glass, taut enough to bounce birds off before they hit. Small-mesh netting (around 5/8″ or 1.6 cm) is best, so that birds don't get their heads or bodies entangled but will bounce off unharmed.

Window Alerts reflect ultraviolet light; when you place the decals around a window, birds will see the ultraviolet light and know there is indeed a surface. Window Alert Decals are the ideal way to prevent birds from accidentally striking windows and harming themselves.