Keel bone damage and access to functional resources and housing areas in laying hens: a systematic thematic review

Keel bone damage (KBD) is a major welfare concern in laying hens; however, evidence on its relationship with access to and use of functional resources and housing areas has not been systematically synthesised. This review examined associations between KBD and perches, resting locations, nests, feeders, drinkers, litter and floor areas, outdoor ranges, popholes, wintergardens, and other defined housing areas. PubMed and Web of Science were searched for empirical studies published between 2006 and 2026 involving laying hens, assessing KBD, and reporting relevant resource- or area-related outcomes. Of 345 records identified, 233 remained after deduplication. Following title and abstract screening, 50 reports underwent full-text assessment, of which 17 were excluded; one additional study was identified manually, yielding 34 included studies. Owing to substantial variation among housing systems, KBD assessment methods, outcome measures, and temporal designs, the findings were synthesised narratively by resource category and direction of association. The evidence base was limited and unevenly distributed, with most studies focusing on perches, elevated structures, and resting locations. Hens with keel fractures often accessed elevated resources less frequently, descended more slowly, or made fewer vertical transitions. However, reduced access did not always correspond to reduced occupancy, as some affected hens remained longer on perches or in nests once these locations had been reached. Evidence for feeders, drinkers, outdoor areas, and ground-level resources was sparse and inconsistent. Few studies established whether changes in resource use preceded or followed KBD, limiting causal interpretation. Overall, KBD was most consistently associated with the use of physically demanding elevated resources, while evidence for other functional areas remained uncertain. Resource occupancy should not be interpreted as unrestricted access because affected hens may reach a resource less often yet remain there longer. Future studies should use longitudinal individual-level designs, distinguish access from occupancy and departure, and combine automated tracking with direct behavioural observation.