The response of avian chewing lice (Psocodea: Phthiraptera) loads to early-1900s urbanization in the Western United States

Abstract Urbanization can alter all types of ecological interactions, including parasitism. For urban-associated hosts, two hypotheses predict the response of parasites to urbanization: dense host populations can increase disease transmission, and/or healthy host populations can increase immunity against disease. Using a historical record of bird specimens, we examined the effect of historical urban environments on parasitic interactions by measuring the abundance of ectoparasites on two bird species: House Finch and Hairy Woodpecker. We counted adult chewing lice and their nits from museum study skins of these two species, which were collected in the western United States from 1900 to 1932. Our urbanness metric for each specimen’s collection site and year was extracted from the Built-Up Footprint Area dataset, which measures the area covered by human structures each decade. Despite finding many lice preserved on museum specimens, we found no significant relationship between urbanness and louse loads. This result could be because lice are not sensitive to their external environment, or because the impacts of urbanization on birds were still small in the temporal range documented by our specimens. We also found that House Finches had more nits but fewer adult Ischnocera than Hairy Woodpeckers, and that Ischnocera and nit loads exhibited slight seasonal variation. Our study emphasizes the value of museum collections for understanding the historical process of urbanization in an ecological context, including bird-louse dynamics, and provides an important initial evaluation of how chewing lice may or may not respond to urbanization.