Effects of Parasitism by a Haematophagous Mite on Reproduction in the Barn Swallow

I studied the effect of the parasitic haematophagous tropical fowl mite (Ornithonyssus bursa, Macronyssidae, Gamasida) on the reproduction of the Barn Swallow (Hirundo rustica, Hirundinidae, Passeriformes). Barn Swallows regularly reuse old nests, but avoid old nests with mites as demonstrated by experimental manipulation of mite loads. An equally large proportion of nests was infected with mites in experimental colonies, where all old Barn Swallow nests were removed, as in control colonies without such removal. Furthermore, the estimated number of mites per nest did not differ between experimental colonies, where all old nests were removed, and control colonies. These results suggest that new nests are not primarily infected from old nests harboring large mite populations. Mite population density per nest after fledging was significantly positively related to mite population density on adult Barn Swallows upon arrival at the breeding site in early spring the same year. Mate choice may significantly affect levels of ectoparasitism, if parasite loads of nests originate from inoculates provided by adult Barn Swallows. In order to mimic the effects of choice of a mate with few or many mites, respectively, I manipulated mite loads of newly built first clutch nests during the egg—laying period of Barn Swallows by either spraying nests with a pyrethrin solution or introducing an inoculate of °50 mites, or nests were simply kept as controls. Addition of mites to nests decreased reproductive success of the birds measured in terms of number of independent fledglings of first clutches and in terms of clutch size, brood size, and number of independent fledglings of second clutches. Barn Swallow nestlings in nests with mites had lower body mass but similar body size compared with nestlings in sprayed nests. Incubation periods were longer and nestling periods shorter for nests with added mites. Interclutch intervals were significantly shorter in the group of sprayed nests, mainly because more such nests were reused for second clutches and because adult swallows suffered less from mite infestations. Slightly fewer Barn Swallows had second clutches during the same breeding season if their nests held many mites. Fitness of adult Barn Swallows is thus directly related to their own mite infection level and that of their mate.