Spider web, unexpectedly important for nest building as they hamper ectoparasites' movements

Because of their fitness consequences for many bird species, the design, materials, and methods used in the construction of nests are shaped by different selective pressures. Nest ectoparasites compromise chick development and survival, hence affecting the fitness of their parents. Many defence strategies have evolved in response, including the use of repellent materials. Spider webs are a common component of avian nests, with their use having been attributed to the fact that they help keeping other materials in place. We explored the hypothesis that spider webs play an antiparasitic role in bird nests. First, we characterised the presence of spider webs in the nests of several urban bird species and confirmed that they are a ubiquitous nest material. Then, we evaluated the effect of spider webs on the mobility of Mallophaga ectoparasitic lice. We found that spider webs significantly reduce the amount of ectoparasites that manage to move across artificial nests and showed that this effect is proportional to the amount of spider web in the substrate. Our results suggest that spider webs play a more generalised function than to provide structural support in avian nests: to control the populations of ectoparasites. Spider web availability depends on a variety of ecological interactions, and it may affect the fitness of birds influencing the status of their populations. Given that the diversity and abundance of insects and of web-building spiders are affected by urbanisation, we propose that the study of such effects will promote our understanding of bird persistence in cities.