Sources of variability in aggregation and sex ratios of Crataerina melbae (Diptera : Hippoboscidae) among adult colonial alpine swifts

Aggregation of Crataerina melbae dies on breeding adult alpine swifts (Apus melba) was low when compared with other host-parasite systems and varied w ith sampling date, year, and sex of the flies. Generalized linear models we re performed to ascertain which factors, extrinsic and/or intrinsic to the host, explained variability in the number of louse flies present on a singl e host, i.e., abundance. Overall abundance was unrelated to any host charac teristic but varied slightly among years. Abundance of female flies varied among years, but also with date of sampling, the number of females increasi ng as the breeding season advanced. In contrast, abundance of males decreas ed as the season progressed, independently of host characteristics. Despite these different patterns, the number of flies of each sex on a given host was strongly intercorrelated. These results suggest that mate attraction ma y explain aggregation patterns in this louse fly species. Overall sex ratio of louse flies did not differ from unity. However, the proportion of males decreased during the breeding season, as a consequence of the opposite sex -related seasonal patterns in parasite abundance. Sex-ratio variability was not related to host characteristics or to infrapopulation sizes.