It is common for birds to mistake the reflection of the sky in a window as a clear path. To help protect birds from accidentally crashing into your windows, apply decals or tinting to reduce reflections. You can also hang wind socks, chimes, mobiles or other objects in front of windows to obstruct birds' views of the reflections.
Right: PAWS Wildlife Veterinarian Dr. John Huckabee gives a thorough medical exam to an injured Northern Flicker who had flown into a window.