How sound is the statement that bird droppings are excused due to the difficulty of avoiding them, especially pigeon droppings, and how can one distinguish between pigeon and raven droppings, and what is presumed upon seeing droppings?

The droppings of birds and animals whose meat is permissible to eat are pure. Therefore, pigeon droppings are pure and do not harm the worshipper if they fall on him. As for the droppings of animals whose meat is not permissible to eat, such as crow droppings, they are impure, but a small amount of them is excused. If bird droppings are mixed and their source is unknown, then if the amount is small, it is excused. If the amount is large, it also does not harm, because the original state of clothes and the body is purity, and impurity is not determined by doubt.