Are common garden birds' poo (pigeons, raven's etc) dangerous to human health if left undealt with on garden furniture?

Not : the best for humans. Would clean all bird droppings the best you can. But it's hard to keep clean all the time. So wash hands and be aware.

Guano (bird poop): Bird poop (or guano) is usually a nuisance more than a health risk, there can be a risk of disease transmission, especially for those with weakened immune systems. Clean up with soap and hot water whenever you have contact with bird droppings. The guano does not present more problems if left for more time on the furniture.