Why Do Hens Pile? Hypothesizing the Causes and Consequences

Abstract

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.

Welfare of broilers on farm

“…In case of piling, typically sudden and frightening events cause the flock to move to one corner resulting in a 'pile' of chicks with the ones in the corner or underneath other chicks possibly smothering to death (Gray et al, 2020). The causing factors and their intensity, e.g.…”

“…The piling itself is not a regular but an extreme event that causes losses. In this turn, the number of dead or culled birds are counted.InterpretationIn case of piling, typically sudden and frightening events cause the flock to move to one corner resulting in a ‘pile’ of chicks with the ones in the corner or underneath other chicks possibly smothering to death (Gray et al., 2020). The causing factors and their intensity, e.g.…”

Welfare of laying hens on farm

“…Panic and smothering events are sporadic and therefore difficult to investigate, but the occurrence of crowding and piling can be monitored. It has been proposed that piling may cause heat stress, injury and breathing difficulties (Gray et al., 2020), all of which would increase overall group stress.…”

“…Hazards for piling and smothering are not well understood. Smothering arises when birds crowd or pile on top of each other causing death by suffocation for hens in the middle or at the bottom of a pile (Bright and Johnson, 2011;Barrett et al, 2014;Gray et al, 2020;Herbert et al, 2021). Crowding and piling can occur relatively frequently and do not always lead to lethal smothering, but smothering is usually preceded by crowding or piling.…”

75. Do we improve any aspects of animal welfare by implementing Computer Vision in livestock farming?

“…Another type of behaviour which causes mortality and harm in laying hen production is the smothering or the suffocation of hens due to crowding and piling. Our understanding of this specific behaviour is limited and additional detailed research is needed (Gray et al, 2020;Winter et al, 2021). These harmful types of behaviour could be found in all housing systems and are more prominent in large groups of hens (Lay et al, 2011).…”