Forced to crowd or choosing to cluster? Spatial distribution indicates social attraction in broiler chickens

We investigated the response of commercially farmed broiler (meat) chickens to their social environment at five stocking densities, using spatial distribution and behaviour. We used a computer model in which a 'social aversion/attraction' parameter was set at different values to give simulations in which the chickens were averse, indifferent or positively attracted to each other. We examined the spatial distribution of real chickens that were neither feeding nor drinking, using video records taken within commercial houses, to see which setting of the model best fitted the observed data. At all stocking densities, chickens were more clustered than indifferent (randomly distributed) chickens, and their distribution best fitted a social attraction model in which simulated birds rejected a potential position if their distance from other birds was too great. The parameter setting that best fitted the observed data was a model in which simulated chickens had a high probability of rejecting a position if the nearest chicken was more than an estimated 75 cm away: they were socially attracted rather than socially averse. This result suggests that, even at high commercial stocking densities, real broiler chickens may find the close proximity of other birds more attractive than aversive. Except for jostling and number of strides per walking bout, behaviour did not change across the stocking densities studied, nor, except for gait, did the most obvious measures of bird health (mortality, culls, leg health). (c) 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.