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.

Window Alert is a static-cling decal that may be applied to home and office windows, helping birds "see" windows and thus avoid striking the glass. The decal contains a component which brilliantly reflects ultraviolet sunlight. This ultraviolet light is invisible to humans, but glows like a stoplight for birds.

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. You can mount the netting on a frame, such as a storm-window frame, for easy installation and removal. One-way transparent film.

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).