Study sites and sampling
All animal collections and experiments were approved by the Committee on the Ethics of Animal Experiments of School of Life Sciences of Lanzhou University. Adult male tree sparrows were caught by mist nets in our study sites, Baiyin (BY) and Liujiaxia (LJX), during their breeding season (April-July) from 2020 to 2022 (Table S1). The polluted site BY has a long history of mining non-ferrous metals since 1956, in contrast with LJX, a tourism city and important water resource protection area that is relatively unpolluted and chosen as control site. The tree sparrow reaches sexual maturity within a year after hatching and breeds every single year in our study sites. The sexes of individuals were determined based on the presence of an incubation patch (Bailey, 1952). Afterward, females were ringed with a uniquely numbered metal band and released, while males were kept from light and transported alive to the laboratory for further experiments. In the laboratory, males were euthanized and dissected immediately. Their tissues were collected and frozen in liquid nitrogen, and then stored at -80℃ until subjected to DNA or RNA extraction.