I've heard of RIRs having them, maybe they inherit the trait from them.

I thought I still had her sickle feathers in my pencil jar, but a quick
look has revealed that they are no longer with the other feather memorials
to birds long gone, so I'm shedding a quiet tear.

--- In smallholding@..., "auntie_mabel" <karence@...> wrote:
>
> Wow, did you manage to get any pictures of her before her moult?
>
> I now have two affected - one is an Isa Brown the other is a Lohmann Brown. I
think I'm