[Maine-birds] Window placement & Bird Collisions - Part Deux

Hi, all.  A week ago I asked about info re windows placement in my re-modeling project in order to avoid bird collisions.  Several people referred me to sites w/ useful info specifically that windows be put in at a slight angle so as to reflect the ground rather than be seen as a clear path.  Problem is - the sites don't provide the actual degree of the angle and my carpenters are understandably uncomfortable w/ such lack of precision.

Does anyone know what constitutes a "slight angle" for this purpose?

Thanks,

Judy Scher