Why are the droppings of chickens, ducks, and geese classified as Heavy (Ghalizah) while other birds are Light?

Physical Impurities: Heavy (غليظة) vs. Light (خفيفة) Classifications

• Because they are land-dwelling domestic birds whose droppings are highly putrid and hard to avoid.
• Because they do not have bones.
• Because their meat is haram.
• Only during the summer.