Breathless Birds: Does air hunger impact the welfare of poultry at slaughter?

Globally, 72 billion chickens are slaughtered each year. Controlled atmosphere stunning (CAS) is increasingly used, involving exposure to gas mixtures causing gradual loss of consciousness, with welfare advantages compared to electrical stunning. Commercial CAS systems most commonly expose conscious birds to increasing concentrations of carbon dioxide (CO2). Although painful levels of CO2 are avoided, these gases cause a hyperventilation (deep breathing) response which is obvious behaviourally. It unknown whether this reaction corresponds to the breathlessness and 'air hunger' reported by humans when inhaling CO2. Air hunger is a potent and deeply unpleasant experience, consisting of an uncomfortable urge to breathe which is associated with profound anxiety, frustration and fear. If the gas mixtures used in gas stunning cause air hunger in chickens, this constitutes a very significant welfare issue, calling into question whether CAS is a humane method of pre-slaughter stunning. Given the anatomical and physiological differences between mammals and birds, the susceptibility of birds to air hunger is difficult to predict, and no previous studies have specifically examined animal perceptions of this sensation. Although some earlier studies have demonstrated that chickens avoid CO2 in certain situations, the available research has never specifically addressed the contribution of air hunger to this avoidance and is confounded by the fact that CO2 is painful to inhale above certain concentrations. Simple behavioural approaches (such as allowing birds to leave a space containing CO2) are flawed since CO2 is an anaesthetic that is likely to reduce the ability to express appropriate responses. More sophisticated behavioural approaches such as those allowing birds to display any learned aversion after, not during, exposure to CO2, and a focus on non-painful concentrations are needed to understand whether air hunger is a welfare issue. The aim of this project is to determine whether birds experience air hunger during exposure to gas mixtures relevant to commercial slaughter.

Initially, we will expose chickens to a range of relevant gas mixtures which represent different CAS methods, untangling their responses to CO2 and low oxygen levels, both of which cause respiratory changes. We will record detailed behavioural and physiological outcomes to determine the conditions under which hyperventilation occurs and how its apparent character and severity relates to gas types and concentrations. We will then investigate whether hyperventilation occurring during CAS represents a welfare issue by evaluating whether chickens avoid and/or escape areas associated with exposure to relevant gas mixtures. We will also deteremine if any spontaneous behaviour corresponds with learned avoidance behaviour to help us understand which apparent signs of breathlessness may indicate unpleasant respiratory experiences. Finally, we will image parts of the brain to examine whether key regions associated with fear, stress and anxiety are activated by exposure to relevant gas mixtures. Collectively, we expect these experiments to allow a corroborative and robust assessment of the welfare implications of hyperventilation.

Tackling the question of whether air hunger is experienced by billions of poultry undergoing CAS is extremely important. The work we propose is ground-breaking and will enable the development of novel methodology to detect air hunger in animals. Whatever the findings, the work will provide valuable information to inform future policy globally. If our studies show that air hunger is not an issue relevant to poultry slaughter, CAS will be proven to be a high welfare method and its uptake will be enhanced. If air hunger is apparent, the results will underpin the development of more humane stunning approaches, ensuring that the current transition away from electrical stunning systems represents genuine welfare improvement.

Controlled atmosphere stunning (CAS) is used in the slaughter of 38% of 72 billion chickens annually, with multiphase systems involving progressive exposure to CO2 being most common. While nociceptive activation in conscious birds is avoided, the extent to which hypercapnic and/or hypoxic stimulation induces unpleasant respiratory experiences is unknown. In humans, hypercapnic exposure is a potent stimulus of air hunger, the most profound and unpleasant form of dyspnoea. Currently, no animal models of air hunger exist. The aim of this project is to determine whether birds experience a negative affective state akin to air hunger during exposure to gas mixtures relevant to controlled atmosphere stunning.

We propose to characterise in detail the ventilatory response of hens to hypercapnia and hypoxia by exposing them to various gas mixtures while fully characterising physiological and behavioural manifestations of ventilatory drive. We will also map activated brain and brain stem regions by quantifying and comparing the expression patterns of the immediate early gene c-fos protein by immunohistochemistry in response to hypercapnic hypoxia, hypercapnic hyperoxygenation, and normocapnic hypoxia. We will determine the welfare implications of the hyperventilation caused by these gas mixtures by evaluating whether hens avoid and/or escape areas associated with exposure in two complementary behavioural paradigms. We will also explore associations between learned aversion behaviour and spontaneously expressed behaviour to validate potential markers of air hunger. In doing so, we will create a novel model for air hunger and robustly assess whether breathlessness is a welfare issue during CAS.
As well as being fundamentally important from the perspective of comparative vertebrate biology, this work addresses a major welfare question affecting billions of farmed birds. Its outcomes will be crucial to inform policy making and the direction of a global industry.