Data from: 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 (C. 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 nest, as these parasites cluster in the lining. We removed the bedding of nests when chicks had recently hatched, and randomly assigned each nest to one of the following treatments: 1) addition of live ticks, 2) addition of dead ticks and 3) simulation of tick addition. Females in the live ticks’ treatment added more butt fibres to their nests than parents in control treatments. Additionally, the amount of butt fibres in the original lining also predicted the amount of fibres added after the manipulation. It seems that the tendency to bring to the nest cigarette butts is at least partially a response to current, and perhaps also past, parasite load.城市野生动物会接触到自然界中罕见的资源，例如城市废弃物中所含的物料。已有多项研究证实，此类资源会对野生动物产生危害。在墨西哥城，家朱雀（house finch）会将烟头带入鸟巢，此举虽能减少巢内的体外寄生虫（ectoparasite）数量，却也会对雏鸟和亲鸟造成遗传毒性损伤。然而，该行为的成因至今仍不明确。 一种假说认为，鸟类从废弃烟头中提取纤维素纤维，仅仅是因为这类纤维的外观与羽毛相似；另一种假说则提出，被拆解的烟头之所以被带入鸟巢，是因为其能够驱避体外寄生虫。 本研究通过评估家朱雀（C. mexicanus）是否会随体外寄生虫负荷升高而在巢中添加更多烟头，对后一种假说进行了验证。当烟头纤维存在时，它们会集中分布于鸟巢衬里中。我们通过移除巢衬，既可清除大部分烟头材料，又能收集到寄生在各鸟巢中的绝大多数蜱虫（tick）——因这类寄生虫常聚集于巢衬内。我们在雏鸟刚孵化时移除鸟巢衬垫，并将每个鸟巢随机分配至以下三组处理：1）活蜱添加组；2）死蜱添加组；3）蜱虫添加模拟组。 活蜱添加组的雌鸟向巢中添加的烟头纤维数量，显著高于对照组亲鸟。此外，巢衬中原有的烟头纤维数量，同样可预测实验操作后新增的纤维量。 由此可见，家朱雀将烟头带入鸟巢的行为倾向，至少部分是对当前乃至过往寄生虫负荷的响应。