Resting or hiding? Why broiler chickens stay near walls and how density affects this

Broiler chickens are reported to be close to walls at higher densities. The reason for this is not clear, since evolutionary theories would suggest that birds should attempt to be in the middle of the flock. We studied the spatial distribution (during weeks 4-6) of broiler chickens stocked at 2.4, 5.8, 8.8, 12.1, 13.6, 15.5, 18.5 and 21.8 birds/m² (in 3.3 m² pens), to investigate the underlying reasons for the pattern of spatial distribution. Three possible reasons were considered: seeking cover from predators in the centre of the flock, seeking cover from predators near walls, and avoidance of disturbances by conspecifics. Spatial distribution was analysed by comparing the number of birds in four separate parts of the pen (inner, inner middle, outer middle and outer). Apart from effects on spatial distribution, birds were predicted to have their behaviour disturbed more often by other birds in the flock as the overall density in the pen increased, leading to shortened bouts of behaviour and an increase in adjustments of the sitting or lying posture. We found that higher treatment density led to shorter sitting and preening bouts (P = 0.024 and P = 0.013), and a sharper decrease in walking bout length as weeks progressed (density × week, P = 0.025). In addition, birds adjusted their sitting or lying posture more often at higher densities (P <0.001), indicating an increased number of disturbances. Preference for the wall area occurred when overall density in the pen peaked (in the last week of rearing, at treatment densities above 12.1 birds/m²). On the other hand, more animals were present in the centre of the pen than at the edges at some of the densities in weeks 4 and 5. Because of its occurrence at peak density, avoidance of disturbance seems the most likely explanation for wall preference. Thus, increased use of the wall area may be an indicator that birds are experiencing crowding.