Because if you have calls that have small beaks and short necks, they struggle to get out of the shell. Some calls do not have this issue, most do. They struggle to breath and there for you get a lot of them that end up sufficating..I don't see why people say hatching call ducks is a challenge! I had 18 eggs to start off with and have ended up with 9 beautiful ducklings!!!! May have another one to go aswell