Why do hens smother? An investigation into the causes and consequences of smothering

There can be no egg farmer who wants to walk into a chicken house and find a pile of dead birds, only to know the same event could happen again tomorrow and the next day, without being able to prevent it. This is the impact of smothering, a strange behaviour whereby hens pile on top of each other causing deaths of chickens at the bottom of the pile. No-one really understands why chickens do this and often they appear to move slowly and calmly in the lead up to smothering. Naturally, farmers who produce eggs would like to reduce this behaviour, but it doesn't happen when they are present in the farm watching the birds. It happens intermittently and is hard to predict. New approaches are therefore urgently needed to understand this behaviour. The behaviour and motivations of birds during these smothering events is of great interest to scientists because animals don't often cause the death of other animals of the same species in such an apparently calm manner. This research aims to investigate when, where and how smothering happens, and the effect smothering has on the chickens that survive. The fact that smothering is unpredictable means that a large number of farms are needed in order to study this behaviour. Working with an egg producing company we will have access to 110 flocks of birds. These all have a monitoring system, Birdbox, which records information about the environment of the hens (e.g. ventilation and temperature), food and water intake, egg production, and numbers and causes of hen deaths. We will also gather further information on whether these chickens were stressed when they were still chicks, details of the barns where they live, at what time smothering events occur and where in the barn they happen. All this information will be used find the potential causes of smothering and to see if there could be early warning signs before it happens. Whilst useful for understanding the development of smothering behaviour, these analyses by themselves won't identify the immediate cues that cause chickens to move in ways which smother others, or indeed what movements cause smothering. To address this we will place heart rate monitors on chickens and film smothering events using thermal and standard cameras. The thermal cameras will allow us to automatically measure movements of chickens to be able to create a realistic simulation of smothering. The simulation of smothering will allow us to understand what it is about hen's movements which means other hens don't move out the way and end up getting killed. Heart rate monitors will enable us to see whether stress might trigger smothering, or whether birds really are as calm as they appear when starting to smother one another. We will also measure other indications of a stressed flock and hen welfare: fearfulness toward humans and novel objects, stress hormones in the egg, faults in eggshells and bone injuries. It is important to understand smothering behaviour because it harms chickens welfare and causes deaths of 1.4% of birds in a flock die from smothering (this equates to 21% of all mortalities) and can kill more than 20% of hens in badly affected flocks. Whilst 1.4% overall may sound small, this equates to over 300,000 hens deaths and 86 million eggs lost across the UK egg farms. The real impact of smothering is probably much greater than this because often hens killed by smothering will not be recognised as such. In addition, the potential for injuries and stress in those that survive have never before been investigated. Our results will be used to understand what causes this behaviour which will help farmers know how to intervene to prevent smothering. Our results will be able to reveal if information in the BirdBox could provide a warning that smothering is imminent, or whether particular movements made by hens might be useful for early warnings of smothering. Through this project we will benefit by understanding the true impact of smothering for laying hen welfare.

Technical Summary

We will undertake the first comprehensive study of smothering in this industry partnered LINK grant. Smothering is an aberrant collective behaviour which causes 21% of mortality in laying hens through piling of birds on top of one another in a concentrated area. We focus on risk factors and mechanisms involved in 'creeping smothers', which occur in open spaces, with slow bird movement and in the absence of any obvious trigger. In an exceptional opportunity, this project will have access to 110 flocks of laying hens equipped with systems to automatically collect real-time data on environmental, resource use and productivity parameters. Additional data will be collected on early life stress and stressors. This data will be used for an epidemiological analysis of risk factors for smothering events (using marginal structure models to estimate historical and current influences). The mechanisms of smothering will be studied using two approaches: simulations of smothering using agent based models; and measuring behaviour and physiology of birds before, during and after smothering events. Flocks which show high levels of smothering will be filmed using thermal cameras, to track chicken movements during smothering events. Trajectories of movements from individual birds will be used to build and test simulations. Chickens in high smothering flocks will also be fitted with heart rate monitors, and after a smothering event, egg albumen corticosterone levels, novel object tests, egg shell irregularities, and keel bone injuries will be measured and compared to age-matched, low smothering flocks. These parallel approaches will reveal local cues of bird movement which result in smothering, the arousal state of birds performing the behaviour, and the welfare consequences of smothering for hens that survive. The project will offer practical and theoretical insights into this welfare concern which costs the UK poultry industry more than £6.5million/yr and 86 million eggs.