You don't say if you have a roof on your pen, but that sounds like a hawk That's what they usuallly do, is skin the neck and head, and
decapitate the bird. Normally we don't have much problem with hawks in the wintertime, but this has been such a mild winter and we had the same type
of weather here today, so maybe you are in the same area? We had a very bad problem with hawks until late in the year last year.
So sorry for your loss. If you don't have a roof on your pen, definitely do that. Hang CD's with shiny ribbons tied to them
around the area where your pen is, so that they flash, spin, and blow. Works well as a hawk deterrent. Putting a roof on will also
help to deter hawks, coons, and any climbing predator In addition, bury hardware cloth or something similar around the base of the pen
it will help to prevent predators digging underneath. Lots of people string a hot wire around the outside of the pen as well.