The effect of piling behavior on the production and mortality of free-range laying hens.

Piling, a dense cluster of hens, is a behavior of major concern to the cage-free egg industry. It can cause large numbers of mortalities at which point it is considered smothering. The aim of this study was to quantify whether piling can also have consequences on production and non-smothering mortalities, which have not previously been described. Additionally, we aimed to describe characteristics of piling behavior relevant to for management. Video footage from 12 flocks of laying hens was analyzed for piling behavior across 3 wks. Production and mortality data were accessed using an integrated online management system. Bayesian linear mixed-effect models were used for formal statistical testing of the relationships between piling and production. Accounting for some missing data, a total of 252 d amounting to approximately 15,624 h were analyzed for the presence or absence of piling behavior, which we believe constitutes the largest analysis of piling behavior in the scientific literature to date. All flocks observed exhibited piling behavior even if they had no history of smothering. On average, flocks piled more than 4 times per day for around 44 min per event and the peak piling time occurred between 1300 and 1359. We found that the number of piling events was associated with a reduction in the number of eggs produced the next day; based on the average of 4 piles per day this amounted to 7.35 fewer eggs per 1,000 birds per day. Contrary to our hypothesis, we found total piling duration per day was positively associated with fewer Grade B eggs, with a decrease of 0.74 Grade B eggs for every hour of piling per day. No relationship was observed between piling and mortality not attributed to smothering. We discuss possible causes and explanations for these results including birds' response to stress, crowd dynamics, and daily rhythms. Here we show the potential for piling behavior to have sublethal consequences on production even in the absence of smothering-related deaths.