UC Davis

Laying hens are increasingly housed in systems that provide opportunities to perform highly motivated behaviors, such as perching. However, if laying hens do not traverse elevated structures successfully, they are at risk for falling from or colliding with perches and other features in their environment. Collisions and falls have been associated with keel bone fractures – a painful injury that results in reduced mobility and lower egg production. Promoting laying hens’ spatial abilities, or how they orient in a three-dimensional environment, may improve movement amongst housing structures and offer a strategy to mitigate keel bone fractures. In Chapter 1, I reviewed the key components of spatial abilities and how they are susceptible to development during early life. This timeframe coincides with the portion of birds’ lives when they are housed in pullet rearing environments, prior to the laying environment. However, the housing systems, timeframes, and metrics evaluated thus far leave limitations in our understanding of how rearing promotes spatial abilities and the consequences for keel bone fractures. Therefore, the objectives for my dissertation were 1) to determine if pullet rearing has a long-term effect on laying hen behavior, 2) to study a specific component of the rearing environment (height) and its influence on behavior and keel bone fractures, and 3) to assess a possible mechanism underlying behavioral differences via neural correlates of spatial cognition. Aviary-reared hens experienced fewer acceleration events and collisions at the keel through peak lay (35 weeks; wk) and displayed preferences for the highest perch through mid-lay (50 wk) compared to cage-reared hens in a laying environment of enriched colony cages (Chapter 2). These findings suggest that hens reared with less environmental complexity need more time to acclimate to layer housing containing structures and display different preferences for height. It was not clear which component of the rearing environment was responsible for these differences. Cage-free rearing environments that primarily differed in available height (floor pen, single-tiered aviary, or two-tiered aviary) also resulted in hens displaying an acclimation period and different height preferences in a multi-tiered layer aviary (Chapter 4). Hens reared with minimal height (floor pen) were initially less active in using an aviary to find a nighttime roosting position, fewer hens used the higher zones of the aviary, and they also laid more eggs on the floor than hens reared with height (single-tiered and two-tiered pullet aviaries; Chapter 4). However, the acclimation period was relatively short as behavioral differences disappeared by the end of the first wk of housing and differences in floor egg prevalence disappeared by the second wk of lay (Chapter 4). Furthermore, cage-free rearing environments did not influence the rate of uncontrolled transitions or the prevalence and severity of keel bone fractures in aviaries (Chapter 4). Keel bone fracture prevalence was high, with  94% of hens experiencing fractures at 30 wk. The majority of fractures were minor in severity. I also aimed to determine if the acclimation period was a result of neurological differences related spatial cognition, as in whether birds reared with minimal height developed different neural capacities for processing spatial information. Neither dendritic morphology nor the gene expression of brain-derived neurotrophic factor in the hippocampus (two proxies for information processing in a brain region associated with spatial abilities) differed amongst cage-free rearing environments at 16 wk (Chapter 3). Taken together, these findings support previous hypotheses that rearing environments affect how hens acclimate to laying environments. The duration of the acclimation period varies by the behavioral, cognitive, and physical abilities promoted during rearing. Providing height during rearing is beneficial when hens are required to traverse height during lay. However, hens reared with minimal height are not neurologically impaired, in terms of the metrics we evaluated, and they acclimate within a few d or wk. The animal welfare implications of this acclimation period need to be assessed on a commercial scale (e.g. accessing feed and water on elevated tiers). Although uncontrolled transitions and keel bone fractures were not affected in the present study, this may be related to the experimental setting of our research. Again, future work is needed in commercial environments where larger social groups and less space for take-offs and landings may exacerbate uncontrolled movements and injuries.