31Low atmospheric pressure stunning (LAPS) is a new irreversible stunning method for broiler chickens 32 (Gallus gallus domesticus), which has the potential to improve welfare during routine slaughter. 33During LAPS, birds are placed in a hypobaric chamber that allows oxygen to be gradually removed 34 from the environment by the controlled removal of air; the staged process takes 280s and reaches final 35 decompression pressure that is 80.6 kPa below atmospheric pressure (nominally 101.3 kPa for an 36 absolute vacuum pressure of 20.7 kPa). In this study, the behaviour of broilers (50 individuals and 50 37 focal birds killed in groups of 20) was observed during LAPS. Latencies, total durations, single bout 38 durations and number of bouts were recorded for all behaviours. Three different decompression 39 curves were applied during the process (based on automatically applied settings related to ambient 40 temperature) and their effects on behaviour were investigated. Not all birds displayed all behaviours, 41 but a subset of behaviours (ataxia, loss of posture, clonic and tonic convulsions and leg paddling) 42 occurred in a consistent sequence. In individuals, mandibulation, headshaking and open bill breathing 43 occurred earliest at 44.5s ± 31.6s, 50.8s ± 38.3s and 57.4s ± 35.8s respectively after LAPS began. 44Ataxia was observed on average at 57.3s ± 11.5s, with birds killed at colder temperatures taking 45 slightly longer to succumb to ataxia than those at warmer temperatures. Loss of posture (LOP) is 46 regarded as a behavioural marker for loss of consciousness and it occurred on average at 80.7s ± 47 17.7s. Clonic and tonic convulsions were displayed after LOP at 110.5s ± 37.6s and 117.4s ± 28.8s 48 after LAPS onset respectively. Mean time to motionless was 199.4s ± 21.3s. The group data were 49 largely similar to that of individuals but were less reliable due to focal birds being obscured by 50 neighbours. Based on LOP, the data suggest that birds are in a conscious state for longer during 51 LAPS than in controlled atmosphere stunning with inert gases, but although the induction to 52 unconsciousness is more gradual, other behavioural responses were equivalent. The occurrence of 53 mandibulation, head shaking, and open bill breathing may be an indication of reduced welfare or may 54 be indications of a non-painful physiological responses to hypoxia in a hypobaric atmosphere. These 55 behaviours occurred at similar levels as seen in CAS with inert gases in poultry and the lack of escape 56 3 behaviours as well as absence of signs of severe dyspnoea suggest that LAPS is a humane approach to 57 stunning of poultry. 58Key words: Animal welfare, hypobaric hypoxia, humane slaughter, and loss of posture, low 59 atmosphere pressure stunning 60 61