Passerine bird reproduction does not decline in a highly-contaminated mercury mining district of China

Mercury (Hg) is a neurotoxic element with severe effects on humans and wildlife. Widely distributed by atmospheric deposition, it can also be localized near point sources such as mines. Mercury has been shown to reduce the reproduction of bird populations in field observations in North America and Europe, but studies are needed in Asia, where the majority of emissions now occur. We investigated the reproduction of two passerines, Japanese Tit (Parus minor) and Russet Sparrow (Passer rutilans), in a large-scale Hg mining district, and a non-mining district, both in Guizhou, southwest China. Concentrations of Hg were elevated in the mining district (blood levels of 2.54 ± 2.21 [SD] and 0.71 ± 0.40 μg/g, in adult tits and sparrows, respectively). However, we saw no evidence of decreased breeding there: metrics such as egg volume, nestling weight, hatching and fledgling success, were all similar between the different districts across two breeding seasons. Nor were there correlations at the mining district between Hg levels of adults or juveniles, and hatching or fledgling success, or nestling weight. Nest success was high even in the mining district (tit, 64.0%; sparrow: 83.1%). This lack of reproductive decline may be related to lower blood levels in nestlings (means < 0.15 μg/g for both species). Concentrations of selenium (Se), and Se-to-Hg molar ratio, were also not correlated to breeding success. Although blood levels of 3.0 μg/g have been considered as a threshold of adverse effects in birds, even leading to severe effects, we detected no population-level reproductive effects, despite ~25% of the adult tits being above this level. Future work should investigate different locations in the mining district, different life-stages of the birds, and a wider variety of species. The hypothesis that bird populations can evolve resistance to Hg in contaminated areas should also be examined further.