Many animals, including insects, mammals, and birds, defecate and urinate away from resting and nesting sites [1
]. Doing so not only reduces the risk of disease [1
], but also prevents drawing in predators [5
]. As a result, many animals choose to forage away from excreta or feces [6
], while some species of birds go so far as to produce fecal sacs that their parents can remove from the nest [4
Given the evolutionary importance of excreta segregation, logic would dictate that domestic birds, such as chickens bred for meat production (broilers), would prefer to avoid soiled environments. Commercial broilers are kept in a single open space with thousands of birds, which can lead excreta to rapidly accrue on bedding during the chicks’ ≈42 day lives. This stocking density limits birds’ ability to separate space into clean and excreta soiled areas [7
]. Standard recommendations for litter (accumulated bedding, feathers, excreta, and waste feed) do not go beyond requiring that litter not be toxic to birds and that it be maintained at an adequate moisture level [7
]. Therefore, animals that are motivated but unable to move away from excreta soiled areas may feel frustrated [2
]. Additionally, soiled litter may inhibit effective preening and dustbathing to clean feathers, which may also lead to frustration [9
Besides the possible frustrations of being unable to escape a soiled environment and developing unclean integument [10
], inadequately managed soiled litter may lead to health concerns. For instance, moist litter increases the risk of contact dermatitis [11
] and increases ammonia production from litter [14
] that can cause respiratory and eye disease [16
]. However, wood-based bedding material alone can cause adverse health effects, including respiratory disease [17
] and dermatitis [20
] in humans. As well, rodent studies have shown that wood-based beddings from softwoods (e.g., pine and spruce) are strongly cytotoxic, while hardwoods (e.g., aspen) tend to be much less cytotoxic [22
] but not as inert as non-wood beddings like corn-cob [23
Yet, while poultry prefer less ammoniated environments [24
], they have also been reported to forage in [26
] and consume excreta [27
] in the absence of other foraging substrates. Although excreta is not a preferred foraging substrate, chickens kept on bedding will consume 5–24% of excreta produced by their group in a behavior known as social homocoprophagy [28
]. One of the possible functions of social homocoprophagy may be to use the vitamins, fats, and proteins contained in cecal excreta. Therefore, this inclination may indicate that, despite increased ammonia concentrations in soiled environments, domestic chickens may be indifferent to soiling. Theoretically then, ammonia reductants could assess birds’ relative preference for soiled litter with reduced ammonia. However, these chemical ammonia reductants act by acidifying litter, and may, therefore, be aversive themselves [29
Preference tests offer animals the opportunity to show their relative preferences for a set number of choices ranging from different floor types [30
] to colors of light [31
]. However, preference tests do not show strength of motivation. To solve this, consumer-demand motivation tests present animals with increasingly challenging obstacles that they must overcome to access a resource; for example, pecking a key [32
], overcoming a barrier [33
], or pushing a weighted door [34
]. Hence, our experiment aimed to assess broiler chicks’ motivation to access four different substrates (fresh pine and spruce wood shavings, fresh aspen wood shavings, soiled pine and spruce wood shavings, and soiled pine and spruce wood shavings treated with an ammonia reductant) compared to their motivation to access feed using weighed push-doors. Furthermore, we used the average maximum weight pushed—or maximum price paid—as a measure of motivation [35
]. This method measures the value of a resource to an animal by evaluating how much work the animal will do to access said resource [37
]. The maximum price paid for the substrates was then compared to broiler chicks’ motivation to access feed, the gold standard of comparison [32
]. Additionally, the chicks’ ability to leave the treatment substrate without paying a cost meant that the reward size (length of visits) was under the chicks’ control. Therefore, we also recorded the time spent and the number of visits to each treatment with varying price. This knowledge would help increase our understanding of what litter conditions chickens prefer and ultimately can help inform farming practices.
Due to the differences in human adverse reactions and cytotoxicity in rodents, we predicted that the chicks would show greater motivation to access fresh aspen over fresh pine and spruce shavings. We also predicted that the chicks would be less motivated to access soiled shavings compared to substrates with reduced ammonia. Thus, we predicted that the chicks would differentiate between the substrates in the following rank order, from most to least preferred: fresh aspen wood shavings, ammonia reductant treated soiled wood shavings, fresh pine and spruce wood shavings, and soiled wood shavings.
This study used a two-compartment choice test to determine broiler chicks’ motivational strength for four different litter substrates (FP, FA, SP, TSP) based on their responses to increased access costs (push-doors weighing 0%, 10%, 20%, or 30% of a pen’s median chick body weight). Their motivation to access litter was then compared to the chicks’ motivation to access feed, the gold standard of comparison in motivation tests [32
]. The chicks’ motivation was assessed based on the maximum price paid—or maximum weight pushed—to access the different resources. We predicted that the chicks would differentiate between the substrates in a rank order from most to least preferred: FA, TSP, FP, SP. We also predicted that increasing the cost to access the resources would reduce the number of chicks’ visits to the treatment compartment (T) as well as increase the time spent in T. Our findings indicate chicks worked hardest for feed and, contrary to our expectations, they did not pay a higher maximum price, spend more time on, or visit one substrate more than the other.
As predicted, broiler chicks paid the highest price for feed, showing feed is a physiological necessity [35
] and that chicks, comparatively, are less motivated to work for bedding and litter (collectively referred to as substrates). These results also align with Dawkins [41
], who found that adult laying hens worked more to access feed over a mix of sawdust and peat moss. The finding that broiler chicks are more motivated to access feed over dustbathing or foraging substrates is also consistent with food- or dustbathing-deprived laying hens, who more consistently chose feed in a Y-maze even when deprived of dustbathing substrates [42
]. Like feed-deprived laying hens, broiler chicks’ disrupted satiety mechanisms [43
]—and therefore genetically increased motivation to eat—makes feed an even stronger motivator [44
] and may further drive a motivation to prioritize feeding over other behaviors. Moreover, broiler chicks’ excessive weight, accompanied by a possible shift in centre of gravity, makes activity tiring [45
], which suggests they might require more motivation to overcome the push-doors to access a resource.
Against our predictions, the maximum price paid for the different substrates did not vary, indicating that the birds were equally motivated to access all substrates. This may be explained by the suitability and attractiveness of the substrates for different activities the bird wished to perform, such as foraging and dustbathing [46
]. However, this study does not report how chicks spent their time on the different substrates and no definite statements can be made based on the current study. Nevertheless, young chicks are motivated to perform active behaviors (such as exploration, foraging, and dustbathing) and rest on substrates. Thus, it is possible that all the substrates were of equal value, being equally stimulating or aversive, or provided equal immediate experience for these behaviors. Additionally, we cannot exclude the possibility that chicks simply worked for additional space to explore [48
]. Similarly, the daily changing of resources (once every 24 h) may have made all the substrates conditionally equally rewarding [49
These results also suggest that chicks did not differentiate between different wood shavings (FA and FP) or litter treated with an ammonia reductant (TSP). Moreover, the chicks were not willing to pay a higher price for unsoiled over soiled substrates, and therefore did not avoid excreta. As previously stated, this could simply be the result of the chicks’ inability to identify any difference between the substrates. Conversely, since animals do not always work for things that benefit them [50
], the chicks may have also simply chosen not to work for them. Yet, many factors could influence the chicks’ perception of soiled substrates. For example, compared to red jungle fowl, commercial chicks have been heavily selected for meat production [51
], which may influence their behavior to avoid excreta. Additionally, these young birds lack maternal care and guidance, which may influence their avoidance of environments similarly to how mother hens influence their chicks’ avoidance and preference for foods [52
]. However, the potential unsuitability of wood shavings for resting, dustbathing [47
], and foraging [55
] may also have reduced the birds’ motivation to access the beddings (FA and FP). Otherwise, the potential aversive aspects of some substrates may have been offset by increased suitability for performing rewarding behaviors, such that the birds responded equally to all of them. For example, broken-down, degraded soiled wood shavings (H litter, SP, and TSP) may be a better dustbathing substrate compared to fresh wood shavings (FA and FP). In particular, friable, used wood shavings tend to have a smaller particle size, and were found to be more stimulating and adequate for dustbathing in laying hens [57
]. At the same time, other aspects of soiled wood shavings (like ammonia for H litter and SP, or acidity in TSP) may have made these substrates less preferable, counterbalancing the rewarding qualities of soiled wood shavings for dustbathing [24
]. Similarly, the higher acidity (average pH of 4.9 ± 2.08) and reduced atmospheric ammonia levels (which is shown by an increase in percent nitrogen in the litter) in TSP litter may have cancelled each other out, rendering TSP no more or less appealing than the untreated H or SP litters. Thus, since it is possible that the birds found both the unsoiled beddings and soiled litters equally unfit, repeating this experiment with a preferred substrate, such as sand [47
], may clarify these results.
At 0% door weight (lifted doors) chicks visited and spent time in all resources a similar amount. Therefore, in the absence of an obstacle, the results for number of visits and time spent suggest that the chicks had an equal relative preference for all resources. At this door weight, the chicks spent an equal amount of time in H and T as they moved between compartments five to seven times per 13.5 h observation period. In this case, the lack of door weight and the absence of footpad dermatitis on the chicks’ feet likely permitted more frequent movement between compartments. Moreover, the additional space offered by the second compartment (T) could be viewed as an enrichment, which may have led to higher activity levels that were beneficial for their health [58
When door weights were applied (10%, 20%, or 30%), the chicks spent more time in H and visited T less, but spent more time in feed compared to the substrates. Therefore, as cost (door weight) increased, the chicks rescheduled their behavior to visit the feed treatment less and increase the duration of each visit. This result is similar to findings in mice and mink [59
]. Moreover, the chicks’ response to increasing door weight suggests that feed outranks other resources for which rescheduling did not increase the duration of visits (time spent).
Similarly to Cooper and Mason [60
], this effect was not observed with all resources, as increasing costs led the chicks to spend less time with the substrates. In fact, the chicks only spent about 10–30% of their time with the substrates when the doors were weighted. This may imply that the benefits of accessing the substrates declined rapidly with time once they were obtained. In other words, the lower amount of time spent on the substrates may reflect the amount of time needed to perform a rewarding behavior on that substrate [60
]. Alternatively, if the chicks moved onto the substrates in T to obtain extra space or distance from conspecifics, then the number of visits and duration of stay in T may be more dependent on individuals. Regardless, this view implies that the chicks’ choice to move between compartments was primarily impacted by their motivation. However, Bokkers et al. [44
] note that broilers are limited more by their physical abilities than motivation, so the chicks’ weight likely also impacted their choices, especially at 10%, 20%, and 30% door weights. Although weight is limited to its effect on broilers’ mobility, as the systematically varied schedule of the different treatments ensured that door weights and resources were not confounded with age or weight of the bird.
Therefore, the simple familiarity of the home litter compared to the other substrates may have influenced their decision to remain in H rather than overcome an obstacle. This is especially relevant to young chicks that may lack the foresight to understand how soiled litter may be detrimental (e.g., in terms of footpad health) or beneficial (e.g., in terms of possible benefits of social homocoprophagy) to their health. Thus, the findings of this research suggest that chickens may be incapable of assessing the long-term consequences of contact with soiled litter [11
], or the potential adverse health effects of pine and spruce bedding [17
]. Additionally, the chicks’ early exposure to increasingly soiled shavings during their first 2 weeks of life may have habituated them to soiled litter before the experimental habituation period. However, the birds’ equal motivation to access the substrates may also be the result of different litter conditions compared to normal farm litter conditions. In particular, the increased airflow of the setup combined with diligent, daily litter monitoring when changing litter may have limited the severity of moisture and ammonia build-up, and thus reduced the aversiveness of H and SP litters. Laboratory analysis of the litters in our study revealed a similar average moisture content for H and SP litters of approximately 29%, which is below the 30% moisture content found to increase the risk of footpad dermatitis in meat birds [61
], a common problem in turkey production. Moreover, maximum ammonia production occurs at litter moistures of 37.4–51.1 [14
], which are higher than the reported average moisture content of H and SP.
Preference tests themselves must also be rigorously designed to ensure that the animals’ response corresponds to the experimenter’s question. While the birds in this experiment were habituated to the substrates, and used a validated operand (pushing a door) [34
] to access each resource, a number of other design factors present limitations to this experiment. It should be kept in mind that this set-up only allowed for testing of one experimental substrate compared to the home litter, and presenting multiple of these substrates at the same time could have influenced birds’ choices. Additionally, the ability of litter to affect air quality around it makes this study a comparison of environments, more so than variables, which makes the task of pin-pointing what exactly birds are motivated to access more difficult [32
]. However, considering the open pen design and well-ventilated environment, this may be less of an issue in our study. Additionally, some preference tests opt to test individual animals [63
] to more clearly observe individual preference; however, we opted to use groups of animals. While chickens moving simply to flock together could affect our findings, de Jong et al. [47
] found that housing hens in isolation made learning the push-door task more difficult. More importantly, chickens are not commonly housed in isolation, so our choice to house in groups could more accurately predict chicks’ choices. However, this experiment is also limited by human perspective, as the few choices presented to the chicks limited what they could or could not be motivated to access. Moreover, interpretation of the chicks’ response to the resources is also limited by a human perspective that views a soiled environment as undesirable, and does not fully understand chickens’ motivations. Animals also have personalities, which means that their motivation to access different options may vary, leading some animals to explore or interact more with the operand amidst a relatively bare environment [64
]. Therefore, animal owners and/or farmers should diligently manage litter conditions, as broiler chicks will not avoid soiled or potentially harmful substrates.