Parasite prevalence differs between an urban and a non-urban population of a songbird, the dark-eyed junco

Background/Question/Methods

Parasite species and infection prevalence vary between host populations.  Because parasites can have major fitness costs for hosts, environments with different parasite communities (eg. urban versus rural environments) may impose different selective pressures on host populations.  Resisting parasites requires energetic investment in the immune system, and adaptation to an environment with high parasite exposure may select for hormonal responses that facilitate increased investment in immunity.  However, differences in parasite-induced selection could interact with other environmental selection pressures to have complex implications for phenotypes. I tested whether differences in parasitism related to differences in hormonal responses between a pair of urban and non-urban populations of the dark-eyed junco (Junco hyemalis), a widely-distributed North American songbird.  I captured 40 adult male juncos from each population (urban: UC-San Diego campus, non-urban: Laguna Mountain, CA) during the breeding season and measured their parasites, immune function, body condition, and hormonal responses to stress.

Results/Conclusions

Blood parasites and ectoparasites were more prevalent in the non-urban population than the urban population (microfilaria - 18% non-urban vs 2% urban; trypanosomes - 6% vs 2 %; hematophagous lice - 28% vs 0%).  These results are similar to previous work comparing urban and rural populations of other bird species, and may be due to reductions in insect vectors in cities.  Coccidia, a fecally-transmitted intestinal protozoan, was more prevalent in the urban population (non-urban 67% vs urban 96%).  The non-urban population increased its level of the hormone corticosterone (CORT) in response to acute stress more than the urban population.  However, relationships between parasitism, CORT, and immune function were not apparent.  Although these populations are geographically close and some non-urban individuals winter in the urban habitat, parasite prevalence nevertheless differs significantly.  Parasitism may be an important variable to consider when comparing urban and non-urban populations.