If you use cold WFP and you come across birds mess:
wet it, give it a good scrub (with bristles, NOT the plastic SIDE of brush), tap the glass using the bristles a couple of times, then wet it again and leave to soak.
Come back to the birds mess and repeat the same procedure again. Eventually it will come off.
You could always fit some white scouring pad to the side/back of your brush (with araldite), to remove the bird mess easier, or better still, use hot WFP as it really does shift stubborn, baked on birds mess a lot easier than using cold WFP.