I usually wait until the deposit is dry and then gently rub the canvas with a soft toothbrush. Usually works. Some people suggest rinsing with clean water afterwards but I don't se ethe need as rain will eventually do the job.
Bird Poop is very corrosive see what it does to car paintwork if left, it eats into the paint so use plenty of water to dilute as best as possible. Pesky birds!!!!
------------- If it ain't broke don't try to fix it!