Human presence outweighs non-anthropogenic factors as a driver of avian nest parasite loads

Human presence and landscape change can alter many aspects of ecological communities and can interact with natural factors to change host–parasite dynamics. In the study reported here, we tested whether an anthropogenic factor, urbanization, affected the abundance and size of hematophagous ectoparasites (swallow bugs: Oeciacus vicarius) found in the nests of cliff swallows (Petrochelidon pyrrhonota). We also examined how swallow bug infestations might be affected by multiple non-anthropogenic biotic and abiotic factors, including season, nest mass, nest colony size, the history of colony occupation, and the presence of other parasitic and non-parasitic arthropods in cliff swallow nests. Our sample included 73 nests collected from 18 cliff swallow colonies in Northern Colorado, USA. Results indicated that swallow bug loads were most strongly predicted by human land use and density, with urban colonies having much heavier infestations than rural colonies. Swallow bugs were more numerous in colonies that were known to have been recently occupied by cliff swallows, and in nests that contained high numbers of other parasitic arthropods, including fleas and ticks. Swallow bug population size did not vary with season, nest mass, or nest colony size, and none of our measured factors predicted swallow bug body length. Results suggest that cities may exacerbate harmful effects of avian ectoparasites by promoting large population sizes, and that these effects may be compounded by the presence of multiple parasitic species in heavily infested nests.