I've used Virosol to break up birds mess before, spray on leave to soak for 5 mins and then wfp with ease
Virosol is a degreaser
Ditto.
Dilute the virosol 1 to 10.
Either spray on the glass or directly to the brush. Just make sure you use high flow rate and rinse off the glass.
Virosol os perfect on bird mess. Combined with a gardiners super lite dual trim flocked brush.
Did this the other day. Easy peasy.