Dear Marie,
  I do not know if there is any extraordinary meaning but a very
practical guess is that if the robin is nesting anywhere in the vicinity
of your house he is acting to protect the nest.  Your obviously very
clean windows allows him to see his reflection and he might think it is
another male and he is pecking it at to chase it away.
  Just my first thought.
See you in the air.
Eleanor Tremblett