The (Cr) gene is a mutation that when homozygous is fatal and most die in the shell.. The skull opens in the embryonic stage and then closes, things dont always work out the best and sometimes the crests are problematic, some ducks will even have more severe problems, neurological. The crest skin, blood, feathers and so on. They can sometimes be injured by pecking from others. Its best to breed a crested to a non crested getting a percentage of cr duklings, breeding 2 ducks which are cr gives more crested ducklings but also more chances of problems.