Frequency of falls in commercial aviary-housed laying hen flocks and the effects of dusk phase length

Even though aviary systems provide a range of benefits for laying hens in terms of animal welfare, falls and collisions have been observed in experimental settings. These falls are likely stressful for the birds and are thought to be linked to the known high prevalence of keel bone fractures in aviary-housed hens. We investigated the frequency of falls and fall patterns such as collisions, height and cause of fall as well as the prevalence of keel bone damage in commercial aviary-housed Swiss layer flocks and the relationship with time of day and week of age. We also examined the effect of a prolonged artificial dusk length on the number of falls. In total, eight commercial Swiss laying hen farms with two barns each were used and dusk length was adjusted to vary continuously between 14 and 76 min over all 16 barns. Video recordings and palpation of the keel bone to assess fractures, deviations and alterations of 100 hens per flock were conducted in week of age (WOA) 20 and 45. Videos were analysed after lights on, at mid-day, during the whole dusk phase as well as after lights off. Statistical analyses were conducted using generalized as well as linear mixed-effects models. Fall frequency was highest during the dusk phase in WOA 20 compared with any other observed times of day and WOA 45 (all combinations: p < 0.001). In addition, the likelihood of falls including a collision was higher at 20 compared with WOA 45 (p = 0.05) and after lights on compared with the dusk phase and after lights off (both p < 0.001). The majority of falls were observed to occur from the top level of the aviary compared with the middle or lower levels (top level: 55.3% vs. middle level: 29.3% vs. lower level: 8.5%). A longer dusk phase did not reduce the number of falls but affected their cause: falls were less likely to be caused by failed landings with increasing dusk length (p = 0.036). The prevalence of keel bone fractures increased from 15.4% in WOA 20 to 64 % in WOA 45 and was not linked with the number of falls. Even though the majority of falls were observed during the dusk phase, prolonging the dusk phase alone did not suffice to reduce falls. Thus, additional factors such as aviary design and management should be further investigated to reduce falls in aviary systems and increase the welfare of laying hens.