I doubt curtains would work, but it might if cloth were flat and close to openings(no ruffles or pleats) and removed when nest are 'open'. Maybe a rod thru top of curtain(sewn 'sleeve') and bottom. Pieces of bamboo might work for rods. Could cover all three nests at once, top rod could hang on a couple nails.Do you think just curtains would be enough to keep them out? And my grandma has a lot of bamboo she just thinned out- would they use that if I went from one side all the way to the other, and higher than the nesting boxes?
Bamboo for roost would have to be about 3" in diameter, maybe bigger depending on span it has to go across. Make sure you remove any sharp bits from branches and might be good to sand surface with rough sand paper so it's not so slippery.