Phenotype alteration causes long-term changes to the social strategies of victimised birds.

Phenotype alterations can occur naturally during the life span of the domestic fowl. These alterations increase the risk to become a target of aggression and may cause a severe impact on the welfare of affected birds. We analysed the behavioural consequences of sequential phenotype alterations and their long-term effects within stable social groups of adult birds differing in group size. Phenotypically homogeneous groups, with 100% or 0% marked individuals, and heterogeneous groups, with 70%, 50% or 30% marked birds, were housed at constant density in groups of 10, 20 or 40. We applied sequential phenotype alterations to homogeneous groups (by marking or unmarking birds) and compared their behavioural response to heterogeneous groups considered controls. Results show that aggression was greatly affected by phenotype alteration but, unexpectedly, group size did not play any relevant role modulating social responses. Aggression was directed towards the first altered birds and was significantly higher than in control groups. Long term effects were detected, as victimized individuals failed to engage in aggression at any time and adapted their behaviour to minimize aggressive encounters (e.g. high perch use). Therefore, we provide evidence of long-lasting submissive strategies in stable groups of adult domestic fowl, highlighting the relevance of phenotype alteration on the social dynamics of affected birds. Phenotype alterations could help explain much of the targeted aggression observed in producing flocks which severely affects animal welfare.