Data from: Host social behavior decreases exposure to vector-borne disease: a field experiment in a “hotspot” of West Nile virus transmission

Animals can decrease their individual risk of predation by forming social groups. The encounter-dilution hypothesis extends the potential benefits of social behavior to biting insects and vector-borne disease by predicting that the per capita number of insect bites should decrease within larger host groups. Although vector-borne diseases are common and can exert strong selective pressures on hosts, there have been few tests of the encounter-dilution effect in natural systems. We conducted an experimental test of the encounter-dilution hypothesis using the American robin (Turdus migratorius), a common host species for the West Nile virus (WNV), a mosquito-borne pathogen. By using sentinel hosts (House Sparrows, Passer domesticus) caged in naturally occurring communal roosts in the suburbs of Chicago, we assessed the risk of WNV seroconversion inside and outside of roosts. We also estimated per capita host exposure to infected vectors inside roosts and outside of roosts. Sentinel birds caged inside roosts seroconverted to WNV more slowly than those outside of roosts, suggesting that social groups decrease per capita exposure to infected mosquitoes. These results therefore support the encounter-dilution hypothesis in a vector-borne disease system. Our results suggest that disease-related selective pressures on social behavior may depend on the mode of disease transmission.