I've had to deal with bird strikes in the past and what I do now is soak it with water/quick detailer (depending on what I have to hand) and once it's reasonably soft, I then "squirt" the poo off. That's either with water from a bottle with a sports cap, or a detailing spray bottle with the nozzle adjusted. Either way, you can produce a narrow high pressure stream to shift the poo.
On the occasions it has been severe/bad, I immediately go to a local petrol station and use their jetwash to clean it off. The longer it's left on, the more damage it will cause.