Effect of group size on dust bathing and reactivity of laying hens in a cage-free system

Cage-free systems, such as aviaries, allow free movements and species-specific behaviours of laying hens, but group size in these systems can challenge the stability of hens’ relationships and their behavioural patterns. Thus, this preliminary study compared the expression of dust bathing (an essential comfort behaviour) at different hours of the day and the response to a novel object (on farm measure of fear) in 1,800 brown laying hens in an aviary when in small groups (225 hens/pen) (at 43 and 45 weeks of age) and soon after grouping in a unique flock (at 46 and 48 weeks). Data were submitted to ANOVA by a mixed model with group size, observation hour (for dust bathing) as main effects, week as a random effect, and pen as a repeated measure. No effect of group size was observed on the total number of animals that touched the novel object during the test. On the other hand, the number of hens on the floor increased from the small groups to the unique flock (17.3 vs 28.1; P<0.001). This could explain the reduction of hens dust bathing (3.21% vs 1.64% of hens in small groups vs flock; P<0.001) likely because of a reduction of the individual space available for this behaviour. On average of observations on the small groups and the unique flock, the number of hens on the floor changed during the day (P<0.001) from a minimum (3.5) in the early morning (05:00) to a maximum (29.6) at 10:00, when most hens had already laid. Moreover, few hens played dust bath in the first day hours (0 at 5:00 to 0.30% hens on the floor at 7:00); dust bathing started to increase around 9:00 (1.41% hens) to reach the highest values at 11:00 and 13:00 (8.09% and 7.48%, respectively). Finally, the rate of hens dust bathing sharply decreased after 15:00. In conclusion, despite hens’ reactivity did not change soon after grouping birds in a unique flock, the reduction of dust bathing in the flock compared to small-groups deserves further investigations along the production cycle