This study focused on the female broiler breeder and behavioural testing was carried out at two specific ages, 4 & 18 weeks. At 4 weeks the experiments included small and large individuals defined as having a body weight in-between 1-2 standard deviations below or above the mean weight of the flock. At the age of 18 weeks the focus was on small and large individuals reared under two different feeding regimens, either being every-day fed or Skip-a-Day fed between week 6 and week 16.

A large variation in body size is present already after 4 weeks, and the common practice on commercial farms is to size sort the hens at this age. The farmer then sorts out the smaller individuals and puts them together in a pen of their own. This is done in order to reduce competition over feed and to facilitate a catch up in growth. As growth heterogeneity develops to such an extent already at 4 weeks of age, this was the primary target age for the present study.

During the rearing phase a common practice in many countries around the world is to use a Skip-a-Day feeding regime between week 6 and week 16. During Skip-a-Day feeding regime the birds are fed the same weekly amount of feed but during fewer days, normally introducing two non-consecutive fasting days (5-2 regime) each week. Skip-a-Day feeding regimes are not allowed in Sweden due to the national animal welfare law, however such feeding regime has been argued to have positive effects on flock uniformity. The reasearch project under which this study was performed had obtained an ethical permit for rearing broiler breeders under a Skip-a-Day feeding regime on a farm level to investigate effects on welfare. Hence, behavioural studies on both every-day fed and skip-a-day fed hens could be carried out. These experiments targeted the age of 18 weeks as this is close to the end of the rearing phase and as the hens then had gone through the whole Skip-a-Day feeding treatment.

For further experimental details, see sub catagories.