Natural Resources, School of


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Ecology, 85(6), 2004, pp. 1619–1626


Copyright 2004 by the Ecological Society of America. Used by permission.


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.