would be my suggestion (a best-guess), to distinguish the hens whose eggs are allowed to develop into full-grown birds from the ones that simply lay eggs (usually referred to in the U.S. as "battery hens," especially if they're part of a large-scale commercial egg-producing business).

As for "head" as a descriptor for poultry, I have a call in to Foster Farms (a major poultry producer in California), but no answer so far (and a complete blank from the fellow on duty at the local grocery's meat department)... I'll keep at it, though.

Cheers, HC