The Relationship between Wintergardens and Smothering

Smothering is a form of mortality which results from groups of hens pressing together in such a way that some hens are killed presumably due to suffocation. A recent Australian epidemiological study reported that smothering accounted for 11% of mortality across 3 separate organisations and in two separate climate zones in Australia (Hemsworth et al., 2022). Previous surveys of producers in the UK reported a positive correlation between range use on a sunny day and smothering and suggested wintergardens (or verandas), by providing a graduation of light intensity from inside to outside and thus encouraging ranging behaviour, may minimise smothers (Rayner et al., 2016). This study utilizes some of the data from the Australian epidemiological study (Hemsworth et al., 2022) to explore the relationships between smothering and the presence of wintergardens in commercial free-range laying hen flocks and to generate hypotheses for future controlled studies.

Data from 24 Hyline brown free-range flocks managed by one large commercial freerange egg organisation were included in this study. Flock sizes ranged from 16,900 to 19,651 hens. All flocks were housed in flat-deck systems (with all feed and water provided on a single level) however 15 flocks were housed in sheds with wintergardens (an enclosed outdoor area with a solid roof) and 9 flocks were housed in sheds with no wintergardens. Hens were housed in these sheds from approximately 17 weeks of age through to approximately 78 weeks of age with access to the range from 22 weeks of age. The number of hens smothered and the location of the smothering event (inside the shed or on the range) were recorded daily by stockpeople for each flock. Wintergardens were considered to be part of the range in the data collection. Data were analysed using a two-sample t-test conducted in R 4.2.1. All data were square root transformed prior to analysis, untransformed means are presented.

Overall, the mean number of hens smothered (per 100 hens) was greater for flocks that did not have wintergardens compared to those that did (X̄_{no wintergarden}=1.86, X̄_wintergarden= 0.94, t=2.89, p=0.01). There was no difference in the mean number of hens smothered indoors between the two groups (X̄_{no wintergarden}=0.38, X̄_wintergarden= 0.41, t=1.10, p=0.28). However, the mean number of hens smothered outdoors was higher for the flocks that did not have wintergardens compared to those that did (X̄_{no wintergarden}=0.90, X̄_wintergarden= 0.31, t=2.11, p=0.05).

A recent study investigating the relationship between sunlight and range use of commercial free-range laying hens found that hens avoid high light intensities (Rana et al., 2022). While the presence of wintergardens would have a clear impact on the light intensity around the pop holes, it is important to note that this was an opportunistic exploration of data collected within a broader research project and the relationship observed between smothering (particularly in the range) and the presence of a wintergarden could be a result of a number of other factors including other environmental factors (such as wind, temperature, predator avoidance) and differences in management practices, which could not be controlled within the design of this study. Further research, using controlled experiments, is required to test this hypothesis on the effects of light intensity around the pop holes on smothering.