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较为罕见的资源，例如城市废弃物中的各类物料。已有多项研究证实，这类资源可能对野生动物产生危害。在墨西哥城，家朱雀会将烟头衔至巢中，此举虽可降低巢内体外寄生虫（ectoparasite）的种群数量，却也会对雏鸟及亲鸟造成遗传毒性损伤。然而，该行为的成因至今仍未明确。一种可能的解释是，鸟类从废弃烟头中提取纤维素纤维，仅是因为这类纤维的外观与羽毛相似。另一种可能性则是，拆解后的烟头被带入巢内，是因为其具备驱避体外寄生虫的功效。本研究针对后一种假说展开验证，通过检测墨西哥朱雀（C. mexicanus）是否会在体外寄生虫负载升高时，向巢内添加更多烟头材料。当烟头纤维存在时，它们会集中分布于鸟巢的巢衬之中。我们通过移除巢衬，既能清除大部分烟头物料，又可收集到寄生于各巢穴的绝大多数蜱虫——因为这类寄生虫多聚集于巢衬之内。在雏鸟刚孵化时，我们移除了各巢穴的巢衬，并将其随机分配至以下三组处理组：1）活蜱添加组；2）死蜱添加组；3）添蜱模拟对照组。与对照组的亲鸟相比，活蜱添加组的雌性朱雀会向巢内添加更多的烟头纤维。此外，巢衬中原有的烟头纤维含量，也可预测实验操作后新增的纤维量。由此可见，朱雀向巢内投放烟头的行为倾向，至少部分是对当前乃至过往寄生虫负载的响应。