most of the time my hot pure water gets them off but sometimes not!today was a typical example....the customer told me about the 2 bird strikes on the back bedroom window...."no problem!"i replied....gave the window an extra scrub and rinse and when i finished she said "one of them is still there!"sure enough i went upstairs to have a look from inside and it looked like id not scrubbed the window at all!
it must be the greasy residue off the birds feathers as it only came off after a squirt of virosol....
also have you noticed that skylight windows take longer than you think to get them clean too?.....