Research note: Impacts of piling behavior on temperature and carbon dioxide in laying hen sheds

Piling, a high density of chickens choosing to gather, is increasingly being recognized as a major problem behavior in the laying hen industry with both economic and welfare impacts. Groups of animals in close proximity generate heat, and observations of piling have noted instances of over 1200 hens in direct contact. Here, we investigate the impact of piling behavior on the temperature of the chicken shed. Since heat stress causes panting in chickens, piling also has potential to increase the CO₂ concentration and as such, we also investigated the impact of piling on CO₂. We used annotations of piling behavior from video footage of approximately 21 days for each of 12 flocks. The Birdbox system for flock management was used to obtain matched logged temperature (°C) and CO₂ (ppm) from two sensor stations every minute, resulting in 17,396 datapoints. Bayesian multilevel modelling was used to estimate the effects of pile number and duration on temperature and CO₂, including an effect to control for time of day. Since baseline daily fluctuations in temperature and CO₂ could not be obtained, time of day effects were modelled in different ways, as autoregressive, random intercept, sinusoidal and polynomial terms. As autoregressive and non-autoregressive models could not be directly compared, we present the results of the autoregressive and best fit non-autoregressive models. We found no association between piling and temperature or CO₂ for the autoregressive models but did find an association between pile number, pile duration and temperature in the random effects model. Higher temperature was associated with an interaction between increasing pile numbers and increasing pile duration. Since the effect size was very small and this result was not replicated in the autoregression model it should be interpreted with caution but does provide interesting rationale for future work investigating behavior-environment interactions.