Birds Flying into Windows - Bird Window Decals to Keep Birds From Crashing into Windows

One of the more upsetting things we have all seen is birds flying into windows. Be it at work in an office building or at home with a picture window, the unmistakeable sound when the bird crashes into a window is one we would all rather forget. But what would cause a bird to go flying into a window at full speed. Are they on the attack or simply daft?
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