A common question I get about chickens is “can they fly?” And to many people’s surprise the answer is yes! Although they might not go far or look very graceful, chickens can absolutely fly. Like their ancestors the Red Jungle Fowl, chickens naturally seek safety by flying up to perch in trees and roost at night. However, the majority of egg laying hens worldwide are kept in conventional cages, without perches or vertical space, preventing flying and other natural behaviors [1]. In recent years, there has been a shift towards cage-free systems such as multi-tiered aviaries, which offer perches, multiple tiers, and elevated nest boxes for egg laying. Although these cage-free systems provide opportunities for natural behaviors such as wing flapping, dust bathing, perching, and flying, they also are associated with a higher rate of injuries [2, 3]. These injuries include painful keel or breastbone fractures which may be due to collisions with aviary structures or failed landings [4]. This led researchers at UC Davis to investigate how we can reduce injuries and better prepare hens for the complexity of cage-free systems.
Young hens, called pullets, start their lives in a separate rearing system for about 16 to 18 weeks before moving to their adult laying environment. Research suggests that the complexity of this early environment, including access to tiers, ramps, and perches, can reduce the likelihood of collisions and keel bone fractures later on [5]. This calls into question the influence that rearing has on hens’ spatial cognition, or their perception, processing, and interpretation of objects, space, and movement.
Various studies have investigated how the environment during this critical period affects the spatial abilities of hens [6-9]. Some studies suggest that a lack of complexity in the rearing environment might impair the birds’ ability to navigate three-dimensional spaces, potentially leading to more accidents in adult housing systems. However, previous work had not investigated how rearing could impact depth perception.
Walk and Gibson designed the visual cliff, a glass topped table intended to evaluate the development of depth perception in infants [10]. The visual cliff has a “shallow” side, where there is a solid surface directly below the glass, and a “deep” side where there is space below the glass. This creates an illusion of a cliff, where it appears you could walk off the shallow side and fall into the deep side. This paradigm has since been used in a variety of species, from goats to turtles, but had not previously been used in adult chickens. Our study modified the visual cliff for hens and used it along with a Y-maze to investigate depth perception acuity in pullets and laying hens from three different rearing conditions [11].
To do this, pullets were reared either on the floor, with a single tier, or with a two-tier structure. The tiered structures included elevated perches and a ramp to allow easy access to the vertical space, while the floor treatment had perches directly on the ground. To then simulate a commercial setting, all birds were transferred to a multi-tiered system with elevated nest boxes at 17 weeks of age.
Birds from each of the three rearing systems were tested on a Y-maze and visual cliff test at ages, 7-8, 15-16, and 29-30 weeks old. For the Y-maze, the birds could choose to exit into an arena through two arms that were either unequal in length (30 vs. 90 cm) or, as a control, both 90 cm in length. This allowed us to examine depth perception on a horizontal plane to see if the birds would take the shortest distance to escape. For the visual cliff, the birds could choose to leave the start perch by stepping to a platform suspended over the visual cliff at various depths, allowing us to look at their response to increased depths.
We found evidence that all the birds, regardless of rearing environment, were able to perceive differences in depth since they exited through the short arm significantly more than the longer arm of the Y-maze. They also responded differently to changes in depth on the visual cliff. The chickens were faster and more likely to cross at the 15 cm depth than the deeper depths, perhaps suggesting more caution with greater depths. All birds looked down more frequently at the 90 cm depth than the 15 cm and 30 cm depths, suggesting a greater level of attention allocated to this depth.
However, we did observe some differences between the rearing treatments. Floor reared birds were less likely to and slower to cross the visual cliff than the other two treatments, especially at 8 and 16 weeks of age. Additionally, the floor reared hens looked down over the cliff more often than the other treatments. This suggested that when raised on the floor, pullets may be more hesitant to cross from a perch to a platform, which could have major impacts for their ability to navigate a multi-tiered cage-free aviary. This is supported by additional research from our collaborators that found that the floor reared pullets from this study were less likely to use elevated space in the daytime during the first week in their adult housing and were less likely to roost on the highest perches for the next two and a half months they were observed [12].
Unexpectedly, we also found an interesting effect of age on performance on the Y-maze and visual cliff. Pullets at 15 and 16 weeks of age took longer to exit the Y-maze and cross the visual cliff than both their younger and older counterparts. Birds at this age also looked down significantly more when compared to 8 and 30 week old birds, particularly at the 90 cm depth. We suggest this may be due to an increase in fearfulness which has been associated with this age group (13, 14), resulting in a greater hesitancy to cross a cliff or explore an arena. This is around the time these birds reach sexual maturity are transitioned to their new, adult housing system, making the increased fearfulness at this age a potential welfare concern. It is important to explore this further to investigate how to reduce fear during this transition and promote exploration.
Our study highlights the potential implications for welfare and management practices, suggesting that floor-reared birds may be less inclined to utilize vertical structures in multi-tiered aviaries. Ensuring early provision of vertical structures like perches and platforms could better prepare pullets for such environments. Although previous research has found that early experiences with vertical structures impacts certain aspects of spatial cognition, such as navigation and memory, depth perception seems to be unaffected. Overall, this study adds to the growing body of literature that providing opportunities to practice natural behaviors such as flying and perching during rearing may increase use of elevated perches in a multi-tiered aviary, potentially resulting in a smoother transition to adult housing.
A special thank you to the Foundation for Food & Agriculture Research that supported this research under award number – Grant ID: 550830.
Claire Jones is an Animal Behavior PhD candidate in the Animal Behavior and Cognition Lab at UC Davis. This research was from her master’s thesis on the impact of rearing environment on spatial cognition in egg laying hens. Claire has continued to research farm animal cognition and is currently investigating cognitive styles in pigs by examining consistent individual differences in learning and behavior.
[Edited by Isabelle McDonald and Nicole Rodrigues]
References:
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