Natural incubation can achieve excellent results but it usually requires some planning and some care during the incubation process. The location in which the broody incubates the eggs must be secure from predators and vermin and isolated from disturbances caused by other members of it's own flock. She must also have a place to exercise and have access to clean water and food whenever she needs it. In addition, the broody must be clean of parasites to begin with and must be kept that way during the 28 day incubation period. That means no lice or mites to torment the broody and then infest the ducklings.
Broodies are prime targets for such pest because they are relatively immobile.
The broody should be sprayed or dusted with an insecticide made for the purpose before she starts to sit and should be treated again on a weekly basis. The legs of chickens should also be watched for any sign of scaly leg mites, which can be killed with oil or kerosene. Remember, a chicken asked to incubate waterfowl eggs must set for a full week longer than would be necessary for her own eggs. She needs an extra measure of care, as a result. Some broodies have better habits while incubating eggs than others.