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.

Vertical stripes or patterns (at least 1/4 inch-wide, maximum spacing of 4 inches), or horizontal stripes (at least 1/4 inch-wide, maximum spacing of 2 inches) are effective at preventing window collisions for most birds (see the last image).

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.