Why do birds fly into windows?

They can't see the glass

In daylight, birds crash into windows because they see reflections of vegetation or see through the glass to potted plants or vegetation on the other side ... There's one additional reason: birds sometimes see their reflection in a window and attack it.

Sometimes birds don't perceive windows as a barrier. Instead, reflections in glass look to them like open space, so they fly full-speed into the window.