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Why Do Hens Pile? Hypothesizing the Causes and Consequences

Lookup NU author(s): Dr Helen GrayORCiD, Dr Rachel Davies, Alexandra Bright, Professor Lucy Asher



This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).


© Copyright © 2020 Gray, Davies, Bright, Rayner and Asher.Piling is a behavior in laying hens whereby individuals aggregate in larger densities than would be normally expected. When piling behavior leads to mortalities it is known as smothering and its frequent but unpredictable occurrence is a major concern for many egg producers. There are generally considered to be three types of piling: panic, nest box and recurring piling. Whilst nest box and panic piling have apparent triggers, recurring piling does not, making it an enigmatic and ethologically intriguing behavior. The repetitive nature of recurring piling may result in a higher incidence of smothering and could have unconsidered, sub-lethal consequences. Here, we consider the possible causes of recurring piling from an ethological perspective and outline the potential welfare and production consequences. Drawing on a wide range of literature, we consider different timescales of causes from immediate triggers to ontogeny and domestication processes, and finally consider the evolution of collective behavior. By considering different timescales of influence, we built four hypotheses relevant to the causes of piling, which state that the behavior: (i) is caused by hens moving toward or away from an attractant/repellent; (ii) is socially influenced; (iii) is influenced by early life experiences and; (iv) can be described as a maladaptive collective behavior. We further propose that the following could be welfare consequences of piling behavior: Heat stress, physical injury (such as keel bone damage), and behavioral and physiological stress effects. Production consequences include direct and indirect mortality (smothering and knock-on effects of piling, respectively), potential negative impacts on egg quality and on worker welfare. In future studies the causes of piling and smothering should be considered according to the different timescales on which causes might occur. Here, both epidemiological and modeling approaches could support further study of piling behavior, where empirical studies can be challenging.

Publication metadata

Author(s): Gray H, Davies R, Bright A, Rayner A, Asher L

Publication type: Article

Publication status: Published

Journal: Frontiers in Veterinary Science

Year: 2020

Volume: 7

Online publication date: 10/12/2020

Acceptance date: 16/11/2020

Date deposited: 14/06/2021

ISSN (electronic): 2297-1769

Publisher: Frontiers Media S.A.


DOI: 10.3389/fvets.2020.616836


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Funder referenceFunder name
BB/T001747/1Biotechnology and Biological Sciences Research Council (BBSRC)
BB/T001747/1FAI Farms Limited