Host selection by the Louse Fly Crataerina pallida, an avian nest ectoparasite of the Common Swift Apus apus

Preferences by parasites for particular hosts may have important implications for the functioning of host-parasite systems, however, this parasitic life-history trait remains little studied. No detrimental effect of Louse Fly Crataerina pallida parasitism has been found on Common Swift Apus apus nestling hosts. Host selection choices may be mediating the effect this parasite has and account for this apparent avirulence. Two aspects of parasite host selection were studied at a breeding colony of Common Swifts during 2008; (1) intra-brood differences in C. pallida parasitism were studied to determine the influence of nestling rank, (2) differences in male and female C. pallida parasitism were investigated, as they may result in varying costs of parasitism to hosts. C. pallida populations were found to preferentially parasitize higher rather than lower ranking nestlings within broods of both two and three chicks. Greater proportions of females were seen upon nestlings than at the nest, and upon higher ranking than lower ranking nestlings within broods. These results indicate that host selection occurs and this may thus account for the lack of parasitic virulence reported within this host-parasite system.

Highlights► The number and sex of Louse Fly parasites feeding on Common Swift nestling hosts was investigated to determine preferences by parasites for particular hosts within broods. ► Higher ranking nestlings harboured more parasites than lower ranking siblings. ► Greater numbers of female parasites were found on hosts than males, indicating sexual differences in parasitism. ► These results indicate that parasites select hosts to increase their fitness. ► No detrimental effect of Louse Fly parasitism has been seen on hosts. Host selection choices by this parasite may be influencing the level of parasitic virulence seen in this host-parasitic system.