Barn Swallows (Hirundo rustica) 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 haematophagous mite (Dermanyssus
hirundinis) in East Asia. For the first time, we found Barn Swallows were
parasitized by D. hirundinis in Asia and, in north-eastern 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 by-product 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.