Tape or decals on the exterior can be effective if you use enough of them to break up the reflection. Studies show that birds will attempt to fly through an area as small as your hand. You really need enough treatments, whatever they are, to ensure birds don't just zip between them (and straight into 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.