Smothering in Commercial Free-Range Laying Hens

‘Smothering’ in poultry occurs when birds mass together, often on top of each other, resulting in death from suffocation (Bright and Johnson, 2011). The small number of reports documenting the incidence of smothering indicate that it accounts for a substantial proportion of overall mortality in free range layer flocks (Barrett et al., 2014; Bright and Johnson, 2011).

In 2019 Australian Eggs Limited funded the Animal Welfare Science Centre and the Veterinary Epidemiology group at the University of Melbourne to undertake a prospective observational study to identify risk factors for smothering in Australian free-range layer flocks.

Three large commercial free-range poultry units were enrolled into the study with data collection activities starting in January 2019. Flocks entered the study on the day of bird placement into sheds and were followed until the day of depopulation. At present, 85 flocks from the three farms have been enrolled with 47 (n = 23 flat deck and n = 24 aviary) of these completing a full production cycle. Data collection for the project is expected to be completed in March 2022. Throughout the project, each of the 51 flocks received a standardised behavioural test to assess the birds’ responses to novelty and humans during the rearing and production periods.

Based on data collected to date, all-cause mortality rate across the three farms was 22 deaths per 100,000 bird-days at risk of which 4.8 deaths per 100,000 bird-days at risk were due to smothering. The frequency of smothering varied across flocks ranging from less than 1 to greater than 20 deaths per 100,000 bird-days at risk. The accumulated project data will be analysed using a counting process Cox proportional hazards regression model where time to smothering death will be the outcome of interest and factors hypothesised to increase daily smothering hazard operating at the farm, shed and flock level (such as behavioural responses to novelty and humans) permitted to change over time.

Our data show that around one in five deaths that occur in free-range layer poultry flocks were due to smothering. Our presentation will provide results of our multivariable analyses identifying flock, shed and farm-level factors associated with increased smothering risk. A novel feature of this project is its size and its innovative approach integrating well-established methods for assessing the behavioural characteristics of poultry with quantitative epidemiological investigatory techniques.