Simple SummaryBroiler chickens are usually housed in high-density, relatively barren conditions that limit their opportunities to express natural behaviors and be active. This study aimed to investigate the effects of complex environments and stocking densities on broiler behavior. The frequency and duration of broiler chicken behaviors, such as preening, dustbathing, foraging, and play were recorded at different time points when birds were 2, 4, and 7 weeks old. We found that environmental complexity positively influenced foraging, locomotion, preening, and overall activity. However, dustbathing and play were not affected by the complexity of the environment. High stocking density resulted in more frequent but not longer activity overall, which suggests disturbances. In addition, high density resulted in less frequent foraging, drinking, and eating. As chickens aged, their activity levels reduced, and they showed potential signs of frustration. However, the benefits of environmental complexity and low density persisted. Overall, providing a complex environment with a low stocking density can enhance broiler chicken welfare by stimulating their natural behaviors.AbstractThe objective was to evaluate the impacts of a complex environment and stocking density on Ross 708 broiler chicken behaviors. Eight pens contained either high complexity (HC) or low complexity (LC) environments, and high (HD) or low (LD) density. Through focal-animal sampling, the frequency and duration of behaviors were recorded continuously for 5 min at two timepoints for one day in weeks 2, 4, and 7. Birds were active for 30% of the observed time, with birds showing more activity in HC compared with LC. Birds in HC pens spent more time preening and foraging than birds in LC pens, which was interpreted as a positive outcome. Dustbathing and play were not impacted by complexity, possibly due to the observation method. Birds were more frequently active at HD compared with LD, but did not spend more time being active, suggesting disturbances. Birds foraged, drank, and ate less frequently in HD compared with LD, presumably because birds had more difficulty accessing resources. Activity and active behaviors reduced as birds aged, while preening frequency increased, possibly due to frustration, but this was not confirmed. Perching was unaffected by age, showing a persistent motivation to perform the behavior. Our results indicate that a complex environment provides positive stimulation for foraging, locomotion, preening, and overall activity. Despite reduced activity, many benefits of the tested environmental complexity and low density persisted as birds aged.