Barn Swallows in East Asia show strong tolerance against nest mites

Hosts defend against parasites through two main strategies: resistance and tolerance. These two strategies have different influences on the parasite load of the host and so have variable effects on the fitness of parasites. In this study, we investigated the strategy used by the Barn Swallow (Hirundo rustica) to defend against a hematophagous mite (Dermanyssus hirundinis) in East Asia. For the first time, we found Barn Swallows were parasitized by D. hirundinis in Asia and, in northeastern China, over 60% of broods were parasitized. We examined whether swallows with better body condition or the males with more extravagant sexually selected traits (longer tail streamers or redder ventral plumage) showed stronger resistance or tolerance to these parasites. The results showed that males with more extravagant sexual ornaments had weaker resistance (heavier brood mite load) than males with less extravagant traits; however, the heavier mite load was likely to be a byproduct of larger brood size. Moreover, the Barn Swallow population showed strong tolerance against the parasite (i.e. the brood mite load did not affect brood size or nestling body mass). It is worth noting that male swallows with redder breast plumage showed weaker tolerance (lower seasonal reproductive success) than other males. Further, we conducted experimental ectoparasite manipulations in 46 Barn Swallow nests to verify the potential effect of these parasites on the body condition of nestlings. Neither the body mass nor the hemoglobin concentration differed between the designed groups; however, the hemoglobin concentration of nestlings decreased with mite load. We concluded that the Barn Swallow population in East Asia used the tolerance strategy, rather than resistance to defend against nest mites, and the ectoparasitic mite did not appear to be a strong selective factor for reproductive success owing to the strong tolerance of the studied Barn Swallow population. Lay Summary center dot To defend against parasites, hosts either reduce parasite burden (resistance) or compensate for parasite damage (tolerance). center dot In this study, we are the first to report the existence of a bloodsucking parasite species Dermanyssus hirundinis on Barn Swallow (Hirundo rustica) nestlings in Asia, and we investigated the swallows defense against this ectoparasite. center dot Different from previous studies which showed that Barn Swallow males with longer tails or redder plumage coloring had stronger resistance against nest mites, our results showed that such males had weaker resistance to ectoparasites in northeastern China, which was likely to be a byproduct of a larger brood size. center dot As heavier mite load did not appear to impair their reproductive success, the studied Barn Swallow population showed strong overall tolerance to these ectoparasites; however, the nest mite could still affect swallow nestling hemoglobin concentration.