Short communication No effect of the ectoparasite Crataerina pallida on reproduction of Common Swifts Apus apus MARK D. WALKER* & IAN D. ROTHERHAM Geography, Tourism, and Environment Research Unit, Sheffield Hallam University, City Campus, Howard Street, Sheffield S1 1WB, UK Previous studies have failed to ascertain negative effects of the Swift Lousefly Crataerina pallida parasitism on Common Swifts Apus apus. Abundances of C. pallida were experimentally manipulated to create broods experiencing either enhanced or reduced parasitism, and host life-history traits were examined. No signifi- cant differences in clutch and brood size, rate of growth, asymptotic and fledging mass and size, or the number of fledglings per nest were observed. Reduc- tions in parasite virulence may have developed due to the connection of C. pallida and host success, or costs may be borne by traits not influencing immediate fledging success. Keywords: Common Swift, louse fly, parasite. By definition, parasitism results in costs to the host (Price 1980). However, studies of parasitism by the Swift Lousefly Crataerina pallida Latreille, 1812 (Diptera: Hippoboscidae) on the Common Swift Apus apus Linnaeus, 1758 (Aves: Apodidae) have found no evidence of negative effects (Hutson 1981, Lee & Clayton 1995, Tompkins et al. 1996). The costs of para- sitism are expected to be high, as C. pallida is an obligate haematophagous nest ectoparasite occurring at high pop- ulation densities (Walker & Rotherham 2010). This lack of parasite virulence is surprising, as the findings of studies of related host–parasite systems have shown clear detrimental effects as a result of parasitism (Bize et al. 2003, 2004a,b, 2005). However, the abun- dance of C. pallida seen in previous studies may not reflect natural abundances. This is because the nests at the colony examined by Lee and Clayton (1995) and Tompkins et al. (1996) were cleaned annually. This is likely to have led to a reduction in parasite abundance, as has been shown in other studies (Møller 1989). The effects of parasitism are thus likely to be reduced due to these lower parasite abundances. The availability of a colonial nesting site with natural parasite abundances offered the opportunity to re-examine this host–parasite system and the level of host resilience to parasitism. This site is similar in nature to traditional cliff vaults used by swifts for nesting. This artificial sub- strate provides numerous nooks and crannies for pupal deposition by the parasite. The natural parasite abun- dance at the colony studied here was greater than that seen by Lee and Clayton (1995) and we assume that parasite pressure was greater. Abundances of C. pallida were manipulated between nests to create broods with either enhanced or reduced parasite loads. Life-history traits associated with repro- ductive success were compared between experimental treatments. Traits investigated included clutch size, brood size, growth rate, and asymptotic and fledging nestling size and mass. Differences in these life-history traits would be indicative of a reduction in host fitness as expected by host–parasite theory. Failure to find such costs would provide support for Lee and Clayton (1995) and Tompkins et al. (1996), who concluded that a reduction in parasite virulence had occurred due to the close synchrony C. pallida and host life cycles. Where parasite fitness becomes linked with the repro- ductive success and fitness of hosts, the development of reduced parasite virulence is expected to be favoured. Lee and Clayton (1995) postulated that the vertical nature of parasite transmission, occurring solely between parent and offspring, meaning parasite dispersal is depen- dent on host reproductive success, accounted for parasite avirulence. METHODS The Common Swift is a migratory insectivore returning to Europe in April to commence breeding. Common Swifts were examined at a colonial nesting site beneath a concrete roadway bridge spanning the Bigge reservoir, north of Olpe, North Rhine Westphalia, Germany (51°04¢00¢¢N, 07°81¢00¢¢E). Nests lie exposed upon the floor of enclosed internal concrete walkways beneath the bridge and are typically situated close to each entry hole Nests are typically widely spaced (mean ± sd = 5.3 ± 4.8 m). Crataerina pallida is an obligate haematophagous hippoboscid nest ectoparasite of Common Swifts. Crataerina pallida and Swift life cycles are closely synchronized. Adults emerge from overwintering pupae each spring. Feeding occurs approximately every 5 days. They are pupiparous, the larvae developing internally before deposition as 4th instar larvae, which pupate immediately (Walker & Rotherham 2010). *Corresponding author. Email: Mark.D.Walker@student.shu.ac.uk ª 2011 The Authors Journal compilation ª 2011 British Ornithologists’ Union Ibis (2011), 153, 416–420