An assessment of normal and abnormal behaviour in farmed poultry

1. Introduction

Throughout human history, approximately 40 livestock species have been domesticated, many of which still contribute to animal agricultural today (Hata et al., 2021). Commercial chicken breeds have been bred specifically for the farm, leading to considerable changes in morphology, physiology, and behaviour (Rubin et al., 2010) and making them the most ubiquitous domesticated animal with an estimated global population of 33 billion (Statista, 2022). But like many species who are farmed at such scale, chickens have developed abnormal behaviours considered by many to be detrimental to their welfare. This is an important area of study due to the sheer number of chickens farmed around the world each year. Furthermore, once maladaptive behaviours have developed, they often become habit-like and thus persevere even if welfare improves (Mason et al., 2004). This paper will explore some of these behaviours, discussing the welfare implications that surround them and, ultimately, what corrective strategies can be implemented to mitigate suffering and promote good welfare.

2. Assessing welfare in chickens

Historically, welfare assessments have focused primarily on evaluations of the physical state, considering indicators such as disease, mortality, and reproductive success. More recently, assessments have evolved to include psychological and affective states. This is especially important when assessing welfare in chickens due to their lengthy domestication process, during which their ability to cope with various types of stressors has been modified (Ericsson et al., 2014). For example, maximum egg laying occurs at 16 hours of daylight per day, so poultry owners routinely install artificial lighting to achieve a higher yield (Ockert, 2019). Similarly, meat-chickens (broilers) have been bred to grow quickly even in sub-par conditions while an average lifespan of just 42 days (Bridgeman, 2021) is not long enough to develop long-term health conditions. Such manipulation demonstrates why physical state alone is an insufficient indicator of welfare in chickens.

3. Broilers

A chicken bred and raised specifically for meat production is known as a “broiler”, selected for their rapid growth rate and high carcass yields (Meluzzi & Sirri, 2009). The vast majority of broilers live on factory farms in populations of approximately 40,000 – 50,000 individuals, offering just 670cm2 per bird (Animal Aid, 2022).

Absence of normal behaviour

Healthy chickens are motivated to perform an array of normal behaviours, including preening, running, scratching, ground pecking, wing flapping, and dustbathing. Being unable to perform these behaviours can lead to frustration (de Jong et al., 2012) and reduced resilience, or ability to cope (Ross et al., 2020). Absence of some normal behaviours can be attributed to stocking densities, with Meluzzi & Sirri (2009) identifying a link between less available space (as birds grow bigger) and reduced motivation to perform normal behaviours. This is supported by Thomas et al. (2011) whose study into the effect of stocking density on broiler behaviour confirmed that birds in “very low” populations shower higher levels (p<0.05) of ground pecking compared to birds in “medium” and “high” populations. Furthermore, foraging behaviour declined as the study progressed. Compassion in World Farming (CIWF) suggest that this is due to deteriorating substrate, which is not typically cleaned out during the broiler’s lifetime, thus discouraging behaviours like dustbathing and scratching (2019).

However, it is important to note that a reduced behavioural repertoire does not necessarily equal poor welfare. The absence of normal behaviour must be considered alongside other factors, including physiological and physical indicators, and the presence of abnormal behaviour.

Abnormal behaviour

Lying behaviour is the most common behaviour seen in fast-growing broilers, who are typically less active that their slower-growing counterparts in higher welfare systems (Bokkers & Koene, 2003). With this comes increased risk of lameness, skeletal disorders, impaired walking, and joint problems (CIWF, 2013), a hypothesis supported by de Jong at al. whose European Food Safety Authority report found that 57% of broilers experience severe walking problems (2012). CIWF (2013) and Rutten et al. (2002) argue that, despite their inactivity, broilers are still motivated to perform normal behaviours but are prohibited by their burgeoning weight. There is also some evidence that suggests tonic immobility could be a causal factor in broiler inactivity. Tonic immobility is a reflex that causes a temporary state of inertia (most commonly seen in the presence of predators) and is generally accepted to increase with fear (de Jong et al., 2012), although more research is needed.

According to the MSD Veterinary Manual (Denenberg & Landsberg, 2016), aggression is one of the most common behavioural problems in chickens, attributing it to stress, overcrowding, and competition for resources. However, this has been refuted by several studies (Pettit-Riley et al., 2002; Mench, 1988) who contend that intense genetic selection for rapid growth has resulted in decreased agonistic behaviours. Although surprising, considering the high-stress environment of the broiler shed, it is fair to argue that broilers have had to radically adapt their physiology and behaviour to maintain rapid growth rates, rendering them too lethargic to perform aggressive behaviours even if the motivation did exist. Further research is needed here.

To conclude, the nature of commercial broiler farming means that stocking densities are high, and emphasis is placed on achieving slaughter weight as quickly as possible. With this comes a reduction in normal behaviour and a significant increase in inactivity, the consequences of which are largely detrimental the broilers’ physical state more so than their mental state.

4. Layers

A ‘layer’ is a female chicken kept primarily for egg production. Through decades of breeding, layers have become increasingly commercially viable, now producing on average 300 eggs per year (a 50% increase on the 1960s) (RSPCA, 2018). According to the British Hen Welfare Trust (BHWT) (2020), since battery cages were banned in the UK in 2012, commercial layers are now housed in one of the following systems:

(i) Enriched cages hold on average 80 hens with a mandatory area for laying, scratching, and perching. Hens remain in these cages until slaughter.

(ii) Barns house hens in large flocks with no access to the outdoors. Hens have a solid, earth floor and access to nest boxes.

(iii) Free-range offers free access to the outdoors and increased environmental complexity.

Absence of normal behaviour

The lack of opportunity for layers to perform foraging, dustbathing, perching, and nesting behaviours is well documented (Manning & Dawkins, 2012; Nicol, 2020; Hansen, 1976). The desire to dustbathe and perch have remained particularly strong characteristics despite decades of breeding, with all hens choosing to perch at night providing enough space is available (Tauson, 2005). Despite this, enriched cages are not required to facilitate dustbathing nor is there a legal minimum size of perching or scratching area (RSPCA, 2018; Nicol, 2020). The same applies to nest boxes leading to increased competition (BHWT, 2020) and the onset of abnormal behaviour.

Abnormal behaviour

Behaviours that arise from an animal feeling frustrated can be unusual and potentially harmful, and are of particular importance to animals confined over a period of time with little to no autonomy over their environment (Manning & Dawkins, 2012). Injurious pecking covers a group of maladaptive behaviours of varying degrees of severity (FeatherWel, 2022), resulting in feather damage or loss, open wounds, cannibalism, and even death. Nicol (2020) defines injurious pecking as “misappropriately redirected foraging behaviour” where the motivation to forage is so strong but the opportunity to do so is so absent, that the behaviour is directed towards nearby conspecifics (Huber-Eicher & Wechsler, 1998 as cited in Nicol, 2020). Where the ability to perform a natural behaviour, like foraging, is thwarted, a stress response is activated as demonstrated in studies like de Haas (2014), who found that serotonin and corticosterone levels corresponded with high levels of anxiety and the onset of injurious pecking. Over time, a compromised homeostasis results in increased susceptibility to disease at a flock level (FeatherWel, 2022) and a heightened stress response with heritable consequences for future generations (Riber et al., 2007; de Haas et al., 2014; Buitenhuis et al., 2004; Newberry et al., 2007).

Lastly, piling is an abnormal behaviour which can lead to smothering (any mortality resulting from a piling event) (Gray et al., 2020). There is very little published literature on this behaviour, partly due to its unpredictable and varied presentation, which Bright & Johnson (2011) have condensed into three categories:

(i) Panic or fear-induced smothers are usually one-off events with high mortality.

(ii) Nest box smothers occur when one hen is using a nest box, stimulating other hens to crowd into the same area.

(iii) Recurring smothers, which have no one causal factor.

While studies into the cause and prevention of piling are limited, fearfulness and panic amongst female chickens is widely acknowledged: in 1976, Hansen attributed nervousness to social pressure (from population densities) and lack of enrichment (from reduced environmental complexity), while panic was ascribed to specific events, such as unusual stockperson behaviour (Richards et al., 2012).

Overall, despite offering 9% more usable space than the battery cages of yesteryear (RSPCA, 2018), enriched housing systems still impose restrictions on movement and the ability to perform intrinsic, highly motivated behaviours. This leads to conflict and frustration, resulting in the onset of abnormal, maladaptive behaviours such as injurious pecking, inactivity, and piling; behaviours that are detrimental at both an individual and group level.

5. Corrective strategies

In layers, abnormal behaviours can be largely attributed to stocking densities and poor environmental complexity, with little to no opportunity for enrichment behaviours. Ross et al. (2020) suggest that improving an animal’s access to preferred resources has the potential to enhance their ability to cope with a life in confinement, where stressful, aversive practices are unavoidable. The outcome of this study confirms the hypothesis, with enriched conditions greatly reducing hens’ stress reactivity to acute challenges. Dixon et al. (2010) support this view by showing that injurious feather pecking could also be reduced with increased enrichment. Currently, the UK poultry industry depends on beak trimming to prevent injurious pecking or cannibalism; a procedure that partially removes the tip of the beak, leaving it blunt or rounded. Despite legislation that mandates how beak trimming is carried out (Mutilations Regulations, 2010), efforts are being made to find alternative methods. For example, research into reducing injurious pecking behaviours through genetics (BHWT, 2022) by asking poultry keepers to be selective about which birds are used for onward breeding (Department for Environment, Food & Rural Affairs, 2018). The DEFRA Code of Practice for the Welfare of Laying Hens and Pullets goes on to state that once good feather coverage has been achieved in two consecutive flocks, beak trimming is no longer required (2018).

The welfare of broilers is overwhelmingly attributed to rapid growth rate and subsequent health concerns. Nevertheless, some research has been carried out into the limitations of broiler housing, which typically lack any appetitive stimuli (BenSassi et al., 2019; de Jong et al., 2012). Mitigative techniques could include enriched feeding, such as scattering pellets to encourage movement; higher quality or more varied substrate to encourage foraging (Arnould et al., 2003); and novel stimuli such as straw bales or wood chip. Such suggestions are supported by studies like BenSassi et al.’s who found that greater environmental complexity corresponded with fewer skin lesions and a lower mortality rate (2019). Secondly, spatial analysis concludes that broilers prefer stocking densities lower than the maximum currently allowed by EU legislation (39–42kg/m2) and are highly motivated to move to areas with lower populations (de Jong et al., 2012). Although a single, optimal stocking density is difficult to pinpoint, Nicol (2020) observes that at densities of 15kg/m2 and above, birds started to experience the proximity of conspecifics as aversive. Overall, broiler farming overwhelmingly contributes to poor physical state and calls into question whether these birds are experiencing “a life not worth living” (RSPCA, 2020). It is important to note that a slower growth rate is the ultimate corrective strategy, and that efforts to improve mental state alone will not be enough to improve overall welfare.

6. Conclusion

The image of a bird in flight is the ultimate metaphor for freedom, and yet an estimated 16 million chickens still live in cages across the UK (McDougal, 2021). Currently, it could be argued that the poultry industry is failing in all three evaluations of welfare. As concluded by de Jong at al. (2012), Bridgeman (2021), and Rutten et al. (2002), broiler management systems are detrimental to their physical state, while Meluzzi & Sirri (2009), Thomas et al. (2011), and Ross et al. (2020) all point towards a lack of naturalness in the lives of today’s commercial chicken breeds. Lastly, with behaviour being the first indication that an animal is experiencing stress, Bright & Johnson (2011), Bokkers & Koene (2003), Nicol (2020), and de Haas (2014) have all identified abnormal behaviours that are indicative of a poor mental state.

It is now widely accepted that chickens are just as cognitively, emotionally, and socially complex as other birds (Marino, 2017), capable and worthy of experiencing positive states. Unfortunately, of the 40 domesticated livestock species used in animal agriculture today (Hata et al., 2021) most, if not all, are denied the autonomy needed to cope with the systems in which they live. Thus, corrective strategies that seek to mitigate suffering and encourage enriched, positive experiences will always be limited.

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