Piling behaviour in Swiss layer flocks: Description and related factors

Smothering is a major concern within the Swiss layer industry as it can lead to a high number of animal losses. The underlying cause is piling behaviour (PB), a phenomenon where hens densely cluster together in the litter area. The aim of this study was to describe PB and events preceding PB. Furthermore, we investigated the relation of the number of piles, pile duration, and number of hens involved in a pile with time of day, flock colour, flock age, environmental factors, and flock responses to behaviour tests. We video recorded the corners of litter areas (floor) inside the barn and winter garden of 13 commercial Swiss layer flocks (5 white, 5 brown, 3 mixed layer hybrids), which were known to previously experienced problems with smothering. We recorded environmental data (air speed, spot temperature) in the observed corners and assessed flock-level responses to two behaviour tests (novel object test, stationary person test). From the video recordings, events preceding piling and piling characteristics were assessed at 20 and 30 weeks of age (w) at three times per day (0−5 h, >5-10 h, >10−15 h after lights on). Statistical analyses included generalized and linear mixed-effects models and Spearman correlations. Results showed that piling events were mainly preceded by single hen activities (77.9%) and non- hysterical mass movements (7.6%). More piles and the largest numbers of animals involved in piling occurred in white and brown flocks at >5−10 h after lights came on. The number of piles was lower and the number of involved animals and pile duration higher in 20 w compared to 30 w. No correlation was found between environmental factors and flock behaviour test responses with piling characteristics. Potential underlying causes for PB are numerous, though we provide and discuss likely mechanisms, including response facilitation, individual stimulus response, and anti-predation behaviour, based on our findings. Furthermore, PB could relate to diurnal behaviours, for example, dustbathing and hens laying floor eggs in the litter area.