Increased detrimental effects of ectoparasites on their bird hosts during adverse environmental conditions.

Parasites are usually assumed to negatively affect fitness components of their hosts, particularly during adverse environmental conditions. We experimentally tested whether the ectoparasitic house martin bug (Oeciacus hirundinis) had larger negative effects on its house martin (Delichon urbica) host during second clutches. Conditions for reproduction decreased during the season, being less favourable for the second clutch than the first clutch. This was demonstrated by smaller clutch sizes, lower breeding success and smaller and lighter nestlings during the second clutch, as compared to the first, in the absence of parasites. Experimental manipulation of the intensity of parasitism resulted in the reproductive success of the host being reduced with increasing numbers of house martin bugs. This effect was much stronger during the second, as compared to the first, clutch. The house martin bug therefore had a greater detrimental effect on the reproduction of its host when conditions for reproduction were poor.