Ventral feathers contained the highest mercury level in brown booby (Sula leucogaster), a pantropical seabird species

Seabirds are extensively used as environmental biomonitors and feathers are among the most analyzed matrices because they are one of the main excretory pathways to detoxify the bird's body of environmental contaminants. Still, there is a variation in contamination level between the different feathers of seabird species, driven by diet and physiology, such as molt strategy and feather formation sequence. We measured total mercury (THg) concentration in different types of feathers (wing, tail, ventral and dorsal) of the same individual in adults and juveniles of brown boobies (Sula leucogaster) from the northern coast of Rio de Janeiro state, Brazil. Brown booby had higher mean THg concentration (μg.g^-1 d. w.) in ventral (adults: 6.46 ± 1.19, 4.79 to 8.34; juveniles: 4.23 ± 0.60, 3.07 to 5.07) and wing (adults: 5.85 ± 1.10, 4.66 to 8.32; juveniles: 3.86 ± 0.54, 3.23 to 4.63), compared to dorsal (adults: 4.52 ± 1.33, 3.01 to 6.44; juveniles: 3.51 ± 0.19, 3.29 to 3.8) and tail feathers (adults: 2.94 ± 0.45, 2.32 to 3.46; juveniles: 2.8 ± 0.23, 2.45 to 3.08). This difference may be explained because feathers grow in a specific sequence during molts leading to different THg concentrations in each type of feather. Additionally, juveniles had significantly lower concentrations of THg than adults in all feather types, which may be explained by the shorter life span, leading to less time to bioaccumulate Hg in their body. It is essential to choose carefully which feather type is more suitable to be used as a biomonitor of THg contamination in a particular species. For brown boobies, we suggest the use of ventral feathers, which represent the highest Hg concentration, are easy to sample and do not impair the seabird's flight ability, although more studies are needed to replicate these results in other tropical seabirds species.