The relevance of variations in group size and phenotypic appearance on the behaviour and movement patterns of young domestic fowl

•We examined the effects of varying group sizes and variability in phenotypic appearance in pullets.•Birds in GS 10 received more aggression; distances travelled were longer in GS 40.•PA affected some behaviours but the effects were challenging to explain.•Behavioural activity and distances travelled declined with age.•Larger groups and enclosures could be beneficial for the welfare of pullets. Variations in the group size of laying hens might increase the risk of undesired behaviours with important consequences for the birds’ health and welfare. However, larger groups housed at constant densities also translate into larger enclosures that may increase space efficiency, therefore improving movement opportunities. The effects of group size (GS), phenotypic appearance (PA) and age on the behaviour and movement trajectories of pullets were analyzed. 1050 Hy-line brown chicks were divided into 45 pens housing 10, 20 or 40 birds at constant density. Simultaneously, different proportions of PA treatments were studied in a full factorial set up (0, 30, 50, 70 or 100% of birds phenotypically modified per group). The PA alteration was achieved by placing a black mark with a non-toxic dye on the back of the birds' head. Birds were observed at 3 age periods during the rearing phase: 5–6 (P1), 10–11 (P2) and 15–16 (P3) weeks of age. The software Chickitizer was used to record behaviour and locations. For the behavioural variables generalized linear mixed models (GLMM) based on a gamma distribution were built, with GS and PA as fixed factors, age as repeated measure and pen as random effect. Some of the variables analyzed presented very low frequencies and non-parametric tests had to be employed. Movement parameters calculated from the recorded positional data were analyzed with normal GLMM models similar to those built for the behavioural variables. The results indicated that birds housed in groups of 10 received more aggression than other GS (P