Effects of rearing with vertical structures on the ontogeny of depth perception in laying hens

Multi-tiered aviary systems provide laying hens with opportunities for species-specific behaviors, including vertical and horizontal movement. However, collisions and failed landings that occur during vertical movement can be associated with injuries. Previous studies suggest that floor rearing of pullets with minimal access to elevated structures may negatively impact adult laying hens' ability to navigate vertical space. However, it is not clear whether this is due to deficits in physical ability or differences in depth perception. This was investigated in pullets and egg-laying hens using a modified Y-maze task and a visual cliff task at 7-8, 15-16, and 29-30 weeks of age. Dekalb White pullets (N = 450) were reared in three different environments until 16 weeks of age: floor, single-tier aviary, and two-tier aviary. From 16 weeks of age, all birds were housed in a multi-tier aviary. The Y -maze task consisted of two arms that were either a ratio of 1:3 or 1:1 in length. Each bird's ability to discriminate the different arm lengths was evaluated based on latency to exit and arm choice. The illusion of depth over a plexiglass floor was tested in the visual cliff task at 15, 30, and 90 cm. Birds were given 1.5 min to either leave the start perch by jumping to a platform suspended over the visual cliff, or step down off the perch to the shallow side. Latency to jump to the platform and frequency of looking down over the visual cliff were recorded. There were no main effects of age or rearing treatment on behavior in the Y-maze, with all birds choosing the shorter arm more than chance (p < 0.001). Regardless of cliff height, birds reared on the floor were less likely to cross the visual cliff at 8 and 16 weeks of age than single and two-tier treatment birds (p = 0.01). However, these dif-ferences were not observed after they were given experience with elevated structures in the adult aviary. The results of the present study suggest that the lack of exposure to vertical complexity during rearing do not cause long-term detriments to depth perception, but could impact successful engagement with elevated heights (i.e., hesitancy to access vertical structures). Further research is needed to determine the timing, quality, and quantity of elevated height exposure that is needed in the rearing environment to encourage sufficient use of vertical space in the adult aviary for cage-free laying hens.