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Colonially nesting Cliff Swallows (Passeriformes: Hirundo pyrrhonota) in southwesternNebraska, USA, are commonly parasitized by hematophagous swallow bugs (Hemiptera: Cimicidae:Oeciacus vicarius) and fleas (Siphonaptera: Ceratophyllidae: Ceratophyllus celsus). We examined towhat degree these ectoparasites represent a cost of coloniality for Cliff Swallows. The number ofswallow bugs per nest increased significantly with Cliff Swallow colony size. Body mass of nestlingswallows at 10 d of age declined significantly as the number of bugs per nestling increased. By fumigatinghalf of the nests in some colonies, killing the bugs, and leaving half of the nests as nonfumigatedcontrols, we showed that swallow bugs lower nestling body mass and nestling survivorship in largeCliff Swallow colonies but not in small ones. Bugs cost nestlings, on average, up to 3.4 g in body mass,and reduced survivorship by up to 50%. Parasitism by fleas showed no consistent relationship withcolony size during the nestling period but increased significantly with colony size early in the season,when birds were first arriving in the study area. Fleas did not affect nestling body mass or survivorshipand thus, unlike swallow bugs, are probably not important costs of coloniality to Cliff Swallows. Fieldobservations and nest fumigation experiments showed that Cliff Swallows apparently assess whichnests are heavily infested with swallow bugs early each spring and select parasite-free nests, leadingsometimes to alternate-year colony site usage. Cliff Swallows were more likely to construct new nests(rather than reusing old ones) in large colonies than in small colonies, probably in response to heavierinfestations of ectoparasites in the existing nests of large colonies.