Empirical measurement of parasite transmission between groups in a colonial bird

While epidemiological models have suggested that the spread of parasites and infectious diseases often depends critically on the extent of movement by infected individuals between populations, there is little empirical information for any organism on the frequency of between-group parasite transmission or how it varies spatially. The transmission of parasites between discrete social groups or populations may also help determine a host's total parasite or pathogen exposure. We measured the introduction of parasitic bugs (Oeciacus vicarius) into colonies of Cliff Swallows (Petrochelidon pyrrhonota) by transient birds from outside each group. The transmission of bugs increased strongly as the size of a nesting colony increased. More total transients visited the larger colonies, and the direction of change in transient numbers and the numbers of bugs introduced at a site from week to week tended to vary together. Transients at large colonies were more likely to have previously or subsequently visited other large, infested colonies. The greater likelihood of parasites being introduced into larger colonies by transient birds contributes to an increase in parasite load with increased colony size in Cliff Swallows.