I've found that the water must be 000 to be effective on bird poo. Even at 002 it isn't as good.
Another point is that seed-eating birds consume large amounts of grit as a part of their diet.
This is taken from Wikipedia:
Gizzard stones
Some animals that lack teeth will swallow stones or grit to aid in digestion. All birds have gizzards, but not all will swallow stones or grit. The birds that do, employ the following method of 'mastication':
"A bird swallows small bits of gravel that act as 'teeth' in the gizzard, breaking down hard food such as seeds and thus helping digestion." (Solomon et. al, 2002).
These stones are called gizzard stones or gastroliths and are usually smooth and round from the polishing action in the animal's stomach. When too smooth to do their required work, they may be passed or regurgitated.
Therfore, don't use a sponge or scrubber on bird poo or you're rubbing the windows with grit.
Let the water do the work and just melt the stuff away.