Flock size during rearing affects pullet behavioural synchrony and spatial clustering

•Behavioural synchrony and clustering in space decreased with increasing bird group size.•Preening was the most synchronised over time and feeding the most clustered in space.•Simultaneous access to resources for all is more important in small than large groups. Animals are often synchronised in their behaviour, with costs and benefits varying according to group size and the behaviour being performed. Making decisions about optimal allocation and distribution of resources to animals in our care therefore poses theoretical and practical challenges. We investigated group size effects on behavioural synchrony and spatial clustering during daytime in pullets of a commercial laying hen strain reared until 18 weeks of age in groups of 15, 30, 60 and 120 (four replicates of each group size). Feeder, drinker, perch and litter space (i.e. floor space allowance) per bird, were constant across group sizes and all resources were continuously available. Even though the absolute numbers of birds performing the same behaviour at the same time or being located together at the same resource patch increased with increasing group size, the relative degree of synchrony of feeding, drinking, perching and preening across the whole flock decreased exponentially with increasing group size (P