An investigation into the host-parasite interrelationship between Common Swifts and Hippoboscid Louse-Flies

Parasitism is defined as an obligatory hetero-specific relationship where resource transfer between a host and parasite occurs. This results in the sub-optimal expression of host life-history traits and a consequent reduction in host fitness. The Louse Fly, Crataerina pallida (Diptera: Hippoboscidae), is a monoxenous haematophagous nest ectoparasite of the Common Swift, Apus apus (Aves: Apodidae). Despite expectations, no detrimental effect to hosts from C. pallida has been determined. Here this relationship is re-apprised. C. pallida life-history is investigated, with particular reference to those traits of pertinence to its parasitic efficacy. Whether C. pallida has a detrimental effect upon A. apus is subsequently investigated. C. pallida was found to exhibit life-history characteristics strongly indicative that it is parasitic in nature. Morphological and ecological adaptations towards a parasitic lifestyle were identified. Higher levels of prevalence, aggregation, and population abundance were observed than previously reported. Populations were discovered to decline over time and to be heavily female biased. Evidence for previously unreported phenomena such as horizontal parasite transmission, intra-brood host selection, population fluctuations, male mating competition, and host facultative heterothermy was discovered. However, no detrimental impact upon a number of host traits, including previously unstudied aspects of nestling post-natal development and parental investment, were ascertained as a result of C. pallida parasitism. Therefore C. pallida does not fulfil the criteria of the standard definition of a parasitic species. The long term intimacy of the association between C. pallida and A. apus may have resulted in the development of reduced parasitic virulence as expected by hostparasite theory. The discoveries made, especially those pertaining to C. pallida population stability and abundance, may have implications for further studies investigating C. pallida virulence. This study emphasizes the need for substantial knowledge of parasitic life-history before the functioning of host-parasitic relationships can be understood. When examining host-parasitic systems the underlying species specific context in which parasitism occurs needs to be considered.