An experimental demonstration that house finches add cigarette butts in response to ectoparasites

Urban species encounter resources that are uncommon in nature, such as materials found in city waste. Many studies have shown that these can be harmful to wildlife. In Mexico City, house finches bring cigarette butts to their nests, which reduces the amount of ectoparasites, but also induces genotoxic damage in chicks and parents. Yet, the reason for this behaviour is unknown. One possibility is that birds extract the cellulose fibres from discarded butts simply because they resemble feathers. Alternatively, disassembled cigarette butts may be brought to the nests because they repel ectoparasites. Here we tested the latter hypothesis by assessing whether house finches Carpodacus mexicanus increase the amount of cigarette butts in their nests in response to a raise in ectoparasite load. When present, fibres from butts are concentrated in the nest lining. By taking it away, we simultaneously removed most of the butt material and collected the bulk of the tick population infesting each nes...城市栖息物种会接触到自然界中较为稀缺的资源，例如城市废弃物中的各类物质。多项研究已证实，此类资源可能对野生动物产生危害。在墨西哥城，家朱雀（house finches）会将香烟滤嘴（cigarette butts）带回鸟巢，此举虽可降低巢内体外寄生虫（ectoparasites）的种群数量，但同时也会引发雏鸟与成鸟出现遗传毒性损伤。然而，该行为的成因目前仍未明确。一种推测认为，鸟类从废弃滤嘴中提取纤维素纤维，仅是因为这类纤维的外观与羽毛相似。另一种推测则认为，鸟类将拆解后的香烟滤嘴带回巢中，是因为这类滤嘴能够驱避体外寄生虫。本研究通过评估家朱雀（Carpodacus mexicanus）是否会随着巢内体外寄生虫负荷升高而增加巢中香烟滤嘴的数量，对后一种推测进行了验证。当香烟滤嘴纤维存在时，它们会集中分布于鸟巢的内衬材料中。我们通过移除这些纤维，既可清除大部分滤嘴材料，也能收集到寄生在每个鸟巢中的绝大多数蜱虫种群。