Probiotics influence mood and social behaviour, and could help manage gentle and severe feather pecking (GFP and SFP) in hens. While GFP is considered beneficial, SFP damages feather cover over time. In previous research, single-strain Lactobacillus rhamnosus reduced SFP under stressed conditions, and increased GFP overall, and multi-strain synbiotics could reduce SFP while providing other benefits. This thesis aims to as-sess the effect of a synbiotic containing Lactobacillus and Bifidobacterium on GFP, SFP, productivity, and body condition in hens. We first conducted a meta-analysis of Bifidobacterium's impact on stress response in mice. The synbiotic was then evaluated by supplementing stressed and non-stressed hens over four weeks. While Bifidobacterium reduced anxiety-like behaviour in mice in response to stress, the synbiotic did not reduce SFP in hens regardless of stress treatment. The synbiotic did change GFP's relationship to stress, which could play a role in hen’s social behaviour and stress management.