Piling behaviour in British layer flocks: Observations and farmers` experiences

Smothering of laying hens, defined as death due to suffocation when hens tightly pile together in layer barns, is a well-known problem affecting the British loose-housed layer industry. However, knowledge about mechanisms contributing to piling in British layer flocks remains anecdotal. To understand piling behaviour mechanisms, the behaviour of 27 British loosed-house layer flocks from two farmer organisations (four brown hybrids, twenty large barns with a flock size of median ± SD, 10 175 ± 5721 birds, and seven small mobile barns, flock size: 2000 ± 0 birds) with a history of piling behaviour was observed. Video observations were taken along walls and in the centre of the floor area of the layer barns. The number of piles, pile sizes, pile durations, events preceding piles, and locations where piles started (wall/floor area) were described for one day (08:30–16:00 h) at three times of day (0–4 h, >4–8 h, >8–12 h after lights on) either around 20 weeks or around 30 weeks of flock age. Events preceding piling were analysed using descriptive statistics. The effects of time of day, flock age, laying hen hybrid, the area where piles started, and colony size on the number of piling events, pile sizes, and pile durations were assessed using univariate analysis and linear or generalised linear mixed-effects models in R. In addition, twelve British farmers, a subset of the investigated farms, were interviewed about their experiences with piling and smothering. Interviews were audio-recorded, transcribed, and analysed using qualitative content analysis. In total, 92 piles were detected in the videos, lasting on average 21.9 ± 29.3 min and involving 25 ± 39 hens. Piles were mostly preceded by the attraction of hens to other hen behaviours (63.0%) and bird movements through high animal densities on the floor area (23.0%). Piles occurred significantly more frequently at > 4–8 h compared to > 8–12 h and 0–4 h after lights on. Pile sizes were larger in the floor area centre than along walls and positively correlated with pile durations and the flock sizes. Interview analysis revealed that farmers considered multiple events to be triggers for piling and smothering, including the transfer from the rearing to the laying environment, flight responses, broken routines, gregarious nesting in nests or on the floor area, and other gregarious behaviours such as dustbathing in the centre of the floor area. They reported that the causes of piling and smothering change throughout the flock cycle and time of day.