Understanding individual range use in free-range chickens - a systematic review

Birgit Szabo, Luc Lens, Joah Madden, Vitor H. B. Ferreira, Frederick Verbruggen

Every year, over 50 billion chickens are raised globally for meat and eggs. Increasing consumer demand has driven a shift towards free-range and organic systems. These systems allow chickens outdoor access aimed to improve behavioural diversity, and consequently, welfare. However, studies show that only a portion of a flock use the outdoor range. What causes these individual differences, the consequences of this variation and potential interventions to improve ranging have become an important research focus especially in the last decade. In this systematic review we synthesise the results of 107 studies on ranging behavior in layers and broilers to highlight patterns and draw general conclusions regarding the factors influencing ranging. Due to the breadth of our focus, we grouped research together covering (i) causes of individual differences, (ii) the consequences of individual differences in ranging for welfare and production, and (iii) interventions that can change ranging behaviour. Overall, some important patterns emerged. On the one hand, environmental factors, such as weather and temperature may predictably affect ranging across the year, particular strains ranged more and ranging increased over time. On the other hand, the results assessing ranging in relation to behaviour and cognition were less clear. Although some evidence showed better spatial cognition in indoor-preferring birds, no clear relationship to fear was evident. While the effect of outdoor access on welfare in layers was rather mixed, the effects on broilers were more straightforward, sometimes indicating better condition in outdoor-preferring individuals. In regard to production, only a few studies focused on the effects of individual ranging level on production traits finding little effect on egg quality in layers and mostly negative effects on weight, but some positive effects on meat quality in broilers. Finally, changes in management, rearing and the structure of the outdoor range show promise to improve ranging behaviour but these come with a financial cost. Overall, many knowledge gaps still remain and for some sections results are based on only a few studies limiting how well we can draw robust conclusions. We provide some suggestions on how to proceed with future investigations. Together, integrating the perspectives presented in this review will help to understand and manage variation in free-range behaviour shown by both layer and broiler chickens.