Mercury contamination is a major threat to the global environment, and is still increasing in some regions despite international regulations. The methylated form of mercury is hazardous to biota, yet its sublethal effects are difficult to detect in wildlife. Body condition can vary in response to stressors, but previous studies have shown mixed effects of mercury on body condition in wildlife. Using birds as study organisms, we provide the first quantitative synthesis of the effect of mercury on body condition in animals. In addition, we explored the influence of intrinsic, extrinsic and methodological factors potentially explaining cross-study heterogeneity in results. We considered experimental and correlative studies carried out in adult birds and chicks, and mercury exposure inferred from blood and feathers. Most experimental

Across avian taxa, the spatial distribution of nests ranges from dispersed and solitary to extremely dense aggregations. Colonial nesting (i.e., aggregated nesting regardless of cooperative behavior; Siegel-Causey & Kharitonov, 1990) has evolved independently several times, yet remains relatively uncommon in birds, occurring in only about 13% of species (Lack, 1968;Rolland et al., 1998), where benefits of breeding in close proximity to conspecifics outweigh potential costs, such as food depletion or increased conspecific brood parasitism (Bonal & Aparicio, 2008;Craik et al., 2018). Benefits of aggregations may include reduced predation due to group defence, increased opportunities for extra-pair copulations, or increased food-finding efficiency (