What they don't mention is that the birds involved were housed by sire family, were close relatives - and the whole process was conceived at the time in terms of kin selection. Check out this quote:
Craig (1982) advocated kin selection, in which pullets are housed in cages together as families with mean performance used as the criterion of selection. He hypothesized that families that perform best tend to have those physiological and behavioral characteristics most appropriate for group well-being and productivity. Muir (1985), Craig and Muir (1993), and Muir (1994) hypothesized that kin selection would favor cooperative tolerant behavior and concluded that selection on family means, when families are kept together as family groups, provides a method of improving traits in which behavioral interactions influence overall well-being and productivity.
The challenge for group selection has always been distinguishing itself from its widely-accepted rival, kin selection. The case of the chickens surely represents a failure to do this.
- Muir, W. M. and Crig, J. V. (1998) Improving Animal Well-Being Through Genetic Selection