Mercury Exposure Risk And Origins Of Night Migratory Birds In Ailao Mountain,Yunnan Province

Mercury（Hg）is one of the most toxic heavy metal in the environment.Its organic form,methylmercury（Me Hg）,has high neurotoxicity,reproductive toxicity,immune system toxicity,cardiovascular toxicity,and renal toxicity,which poses a potential exposure risk to humans and wild animals（such as fish,birds,etc.）through the bioaccumulation and biomagnification in food chains.Birds are general predators at the top of the food chain,which is sensitive to Hg toxicity in the environment and can accumulate extremely high levels due to the bioaccumulation and biomagnification of Hg.Numerous previous studies on Hg exposure to birds have mainly focused on resident birds with few on migratory birds.Ailao Mountain（Mt.Ailao）,the National Nature Reserve in Yunnan,is a flyway of migratory birds to overwintering places,and a large number of migratory birds fly through Mt.Ailao during their annual migration.To date,however,the population,Hg exposure,and potential origins of those migratory birds passing through Mt.Ailao is not clear yet.In this study,the nocturnal migratory birds passing through Mt.Ailao,Jinshan Yakou Banding Station of Zhenyuan Management and Protection Bureau in Yunnan were investigated to elucidate total Hg（THg）and Me Hg as well as their origins.Concentrations of THg,Me Hg,as well as stable isotopes of carbon(δ¹³C),nitrogen(δ¹⁵N),hydrogen（δD_f）in feather samples of night migratory birds were analyzed for investigations on the accumulation of both THg and Me Hg as well as its factors influencing.Simultaneously,potential origins of dominant population migratory birds,Siberian Rubythroat,was discussed based onδD_f,combined with global atmospheric precipitation hydrogen isotope（δD_p）.The main results are as following:（1）A total of 2,995 feather samples of birds were collected from 2018 to 2020 at Jinshan Yakou Banding Station,Mt.Ailao,Yunnan Province,belonging to 11 orders,23 families,and 114 species,of which mainly passerines with a total of 2132 accounting for 71.2%of the total.Songbirds,waders and climbing birds were the main ecological groups.Brown Shrike,Siberian Rubythroat,Siberian Blue Robin,Thick-billed Warbler,Cuckoo Small and Chinese Pond Heron are six dominant species,of which Brown Shrike ranked first for 3 consecutive sampling years.（2）Values ofδ¹³C in feathers of nocturnal migrating birds in Mt.Ailao,Yunnan province,ranged from-27.2‰to-16.7‰,with an average of-23.8±1.01‰,and the Ruddy-breasted Crake exhibited the highest value,while the Black-crowned Night-Heron the lowest.Values ofδ¹⁵N ranged from 2.25‰to 12.3‰,with an average of 6.95±2.07‰,and the Ruddy-breasted Crake exhibited the highest value,while the Cuckoo Oriental the lowest.Significant differences of bothδ¹³C andδ¹⁵N among bird populations were observed,which probably attributed to their different habitats and food sources.The high values ofδ¹³C found in Ruddy-breasted Crake probably related to its high ratio of C4-plant diet.（3）Concentrations of THg in feathers of nocturnal migratory birds in Mt.Ailao,Yunnan province,ranged from 0.0563 to 15.7 mg/kg with an average of 1.29±1.45 mg/kg,Me Hg ranged from 0.0274 to 14.1mg/kg,with an average of 0.856±1.22 mg/kg,and ratios of Me Hg to THg（Me Hg%）ranged from 10.5%to 100%with an average of 57.7±22.3%.In ecological groups,waders have the highest THg and Me Hg,followed by raptors,climbing birds,and songbirds,and land birds are the lowest.In taxonomic affiliation,Ardeidae are the highest,and Columbidae the lowest.In different feeding habits,fish-eating birds are highest,and herbivorous birds the lowest.Positive correlations betweenδ¹⁵N and THg（r=0.50,P<0.01）,Me Hg（r=0.50,P<0.01）,implied that the increasing trend of Hg with trophic levels increase of birds.（4）Values ofδD_f of Siberian Rubythroat,Black-crowned Night-Heron,Chinese Pond Heron,and Brown Shrike from Jinshan Yakou of Mt.Ailao,Yunnan province exhibited a large range of-102～-28.0‰,with an average of-64.1±15.0‰（concentrated range of-80.0～-40.0‰）.The possible breeding grounds of Siberian Rubythroat passing through Mt.Ailao,Yunnan province were Russian Far East（δD_f:-102～-80.0‰）,and in China,Heilongjiang（δD_f:-80.0～-60.0‰）and Sichuan（δD_f:-60.0～-50.1‰）according to their feathers’δD_f combined with the spatial distribution of global atmospheric precipitationδDp.