This very same thing happened to a friend several years ago.  We both think that
when the light is just right, the reflections read "space" not "glass".  

There's a hill behind our house, and sometimes birds crash when the sun shines
onto the hill and gets reflected in the glass facing the hill.  We have a number
of photos of stunned birds in our hands, "catching their breath", before they
can take off again.  solitary vireo, titmice, chickadees etc..... and there's
some that don't recover.

Susan J.
Norwich

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