A paper in the Journal of Evolutionary Biology was published, reporting analyses of survival in primarily nestling birds from the 1980's, in which we found that ectoparasitic swallow bugs apparently drive stabilizing selection on intermediate nestling body mass and brood size in cliff swallows. In the presence of bugs, nestlings reared in the more common brood sizes of 3-4 nestlings and with intermediate measures of body mass were more likely to survive their first year (to return the next summer) than were nestlings reared in larger or smaller broods or ones of heavier mass. In contrast, when bugs were experimentally removed by fumigation, the heaviest nestlings survived best and brood size had no effect on survival. The results are the first to show an effect of an ectoparasite on the evolution of avian brood and body size.
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