In comparison to the extensively documented mercury (Hg) biomagnification in food chains of aquatic/or aquatic-related systems, Hg biomagnification in food chains in strictly terrestrial systems is poorly explored. Here, we report Hg biomagnification through food webs in a monoculture subtropical pine forest in southwest China. A clear pine needle-caterpillar-tit nestling food chain was determined with the help of live nest videography observations combined with stable isotope analysis (SIA). Simultaneously, a potential pine needle-herbivorous/omnivorous insect-mantis/lacewing/spider food chain was identified by SIA. The results verify that dietary composition plays a pivotal role in the Hg biomagnification in food chains. Distinct total Hg (THg) and methylmercury (MeHg) biomagnification through the determined and potential food chains was observed, with quite similar efficiency of Hg biomagnification based on the trophic magnification slope (TMS) (TMS_THg was 0.18±0.03 and TMS_MeHg was 0.36±0.05 for the determined food chain; and TMS_THg was 0.18±0.04 and TMS_MeHg was 0.38±0.07 for the potential food chain). The TMS values were significantly higher than those from freshwater studies in tropical regions (0.12±0.12 for THg and 0.16±0.07 for MeHg) and forest studies in temperate regions (0.20-0.28 for MeHg). We highly recommend live nest videography observations combined with nitrogen isotope analysis when specifically assessing the biomagnification of environmental pollutants through food webs involving nestlings of free-living birds.