Birds carry a variety of diseases such as Ornithosis, Listeria and E-coli that can be transmitted to man not only from the droppings but also the birds themselves. When dry, pigeon droppings can become airborne in small particles, which can lead to respiratory complaints such as psittacosis.
Bird droppings are acidic and can corrode/erode metals, stonework and brickwork. Nesting materials birds use can block chimneys, flues and guttering, causing possible issues with carbon monoxide and damage to buildings as water overflows from blocked gutters -
BPCA