Social structure and interactions of laying hens: Limited differences between small and large groups

Abstract

As the egg-laying industry transitions from cages to cage-free housing, laying hens are increasingly housed in large groups. Cognitive limitations may impair recognition of conspecifics in such large groups, raising questions on the hens’ ability to form dominance hierarchies as in natural settings. While some studies suggest reduced aggression in larger groups and potential changes in assessment of conspecifics, systematic investigations of social structures and its impact on individuals are lacking. Therefore, we assessed social interactions and structure of 418 White Leghorn hens divided into six pens (three small groups of 20, three large groups of ∼120). From 10–12 and at 24 weeks of age (WoA), we observed social interactions across all hens within a pen and employed Elo ratings, a ranking approach suited for large groups, to assess dominance rank. Furthermore, using 14 focal hens from each group (N = 84), at 16 WoA we tested fear responses (latency to emerge; choice between food and conspecific) and at 26 WoA assessed recognition (choice between flockmate and non-flockmate) and measured body weight and comb size. Contrary to expectations, social structures seemed to be consistent across small and large groups. Within each group, steepness (>0.79) and transitivity (>0.70) were high, indicating hierarchical structures across both group sizes. Top-ranking individuals accounted for over 70 % of interactions in both group sizes and aggression followed a downward heuristic, demonstrating uniform aggression against lower-ranking individuals irrespective of group size. Group size did not significantly affect the number of aggressive interactions per individual or the fear responses. We also found no significant effect of weight or comb size on the rank in either group size. However, in the recognition test, hens from large groups tended to spend more time with the first individual encountered, regardless of flock-origin, whereas hens in small groups preferred the flockmate, suggesting that recognisability only influences social decisions in small groups. Taken together, our findings challenge the current notion of reduced aggression in large groups of chickens and suggest that hierarchical structures exist in large groups. The observed social dynamics may pose challenges for low-ranking hens in large flocks, due to the pecking pressure exerted by increased numbers of higher-ranking hens.