Influence of space availability on measures of fear in conventional broiler chickens

Many of the fear tests used in the welfare assessments of broiler chickens assume that birds will withdraw from a given stimulus if they are fearful. However, fear test responses may also depend on the amount of space available in the broilers’ environment. As broiler chickens grow, the available space decreases, which, together with a decline in walking ability, may bias the results of fear tests. In the present study, we aimed to examine the relationship between space availability and measures of fear in broiler chickens. We hypothesized that broilers kept in pens with lower space availability would show reduced withdrawal in two movement-dependent fear tests, the stationary person test (SPT) and the novel object test (NOT), as space restrictions would limit the birds’ ability to move away. Fast-growing broilers (Ross 708) were housed at similar target stocking densities (34kg/m2) until 27 days of age, after which an experimental intervention was introduced in which half of the pens were provided with additional space, reducing the target stocking density to 16kg/m2. The fear tests were conducted when the birds were 21, 27 (before the change in space availability), 29, and 37 days old. Statistical analyses were performed in R using Generalized Additive Models. We found no effect of space availability on the measures of the SPT (P = 0.32). Space availability seemingly had a temporary effect on the measures of the NOT, as birds housed in the larger pens (16kg/m2) showed increased withdrawal compared to the birds housed in the smaller pens (34kg/m2) at 29 days of age (P = 0.04). However, no difference was found at 37 days of age (P = 0.63), and, therefore, we speculate whether the temporary effect at 29 days of age was due to the novelty of the environment rather than due to increase in space availability per se. We conclude that space availability had no effect on the outcome of the SPT and that it, overall, had no lasting effect on the outcome of the NOT. We speculate whether our findings might be explained by our chosen levels of stocking density for the smaller pens, as these may not have been sufficiently high to limit the birds’ ability to respond to the fear tests used, i.e. the birds, although space restricted, still had the option of being outside of proximity of the presented stimuli.