It occasionally happens to us because we have a bird table with a roof on our patio and the two most open sides face the house and into the garden. because this gives us the best view of them. However, occasionally when the birds are startled they fly off hurriedly out of one of the openings point into our patio window. (Normally it's only the daft collared doves which do this). You can get cut-outs to stick on the windows but they are rather disfiguring and net curtains may help, but again they do block your views. If you feed your birds, can you move the feeders further away from the house to reduce the risk. Also, sometimes birds fly at the corners of the windows to catch spiders in the their webs and this may be the cause of a few accidents.