ECTOPARASITE LOADS AFFECT OPTIMAL CLUTCH SIZE IN SWALLOWS

The optimal clutch size is usually defined as that which gives rise to the largest number of recruits. In birds it is often estimated from a second order polynomial regression of brood size on clutch size. If loads of ectoparasites differentially affect the reproductive success of differently sized broods, for example, because of density-dependent population growth of parasites related to host clutch size (i.e. larger clutches incur larger parasite populations), then optimal clutch size of hosts should vary inversely with parasite load. I tested this in the case of the haematophagous mite Ornithonyssus bursa (Berlese) and its swallow Hirundo rustica L. host by including an interaction term between parasite load of nests and clutch size in a regression model. O. bursa reduced the reproductive success of their swallow hosts in a number of ways. Mite loads significantly reduced the most productive clutch size of the swallow (i) in unmanipulated broods, (ii) in broods where parasite loads were experimentally manipulated, and (iii) in broods where both clutch size and parasite loads were experimentally manipulated. Female swallows were apparently unable at the start of egg laying to predict the mite load of their nests at fledging, since clutch size was not significantly correlated with parasite loads. Females were therefore unable to track fluctuations in optimal clutch size created by parasites, but since mites had greater effects on larger clutches this would reduce the geometric mean fitness by swallows laying large clutches.