This sounds like an obstruction or abscess. In either case, this is not normal and not due to mud. The swan needs to be taken to a licensed veterinarian with swan/ waterfowl or avian (bird) expertise. An xray should be taken. If an object present, then surgery should be done to remove the object. If an abscess or object, the bird should immediately placed on broad spectrum antibiotic and culture taken. Once results of culture are rendered, the exact antibiotic can be prescribed. Once the swan is placed inside for surgery or antibiotics, it may lose weight and stop eating or have minimal appetite. Supportive care should then be rendered such as supplemental tube feedings, human protein powder or supplemental critical food products for birds. A vitamin B-12 shot given to enhance appetite. Only one shot given of the vitamin. Good luck and let us know how the swan is doing. The Regal Swan