Phage,Re lethal traits - there are multiple genes contributing to crest.
The crest interferes with the sutures in the skull and can change the shape of the skull - or even leave a defect (as in silkies). It could be that if too many genetic influences for a large crest could cause a lethal deformity and reduced hatch rate in these chicks.
With swedish flower hens it is il-advised to breed crested to crested as if will produce a huge crest and I believe lead to a reduced hatch rate for this reason. I certainly used to have some dead in shell when I was hatching the swedish flower hens. The same goes for silkies.
That said I have no problem at all getting CL eggs to hatch, and very very rarely have a dead in shell. But then my birds have normal sized crests- not huge or tiny (and I have an idiot-proof bator).
thanks for the insight... someone who hatched from eggs I shipped - had a domed skull female. Another person found incomplete skulls in eggtopsey. I think I may have a female here with recessive domed skull -- that when line bred will produce skull problems -- I guess it is time to check her into the "eggs only" squad and eliminate her from breeding.
Hens go broody when you don’t want them to… and won’t go broody when you do. 
