Elevational Patterns of Blowfly Parasitism in Two Hole Nesting Avian Species

Abstract

Climate change is predicted to cause shifts in parasite distributions, leading to encounters with new hosts. Mountains offer a natural experimental background to study how parasite distributions vary across climatic gradients. Parasite abundance is generally assumed to decrease with ascending elevation, as colder climates may preclude several parasites to complete their life cycles. The present study analyses the elevational variation in the prevalence and intensity of the blowfly Protocalliphora azurea found in the nests of two hosts—the coal tit (Periparus ater) and great tit (Parus major)—in Sierra Nevada (SE Spain). Protocalliphora azurea adults are free-living flies, while their larvae are nest-dwelling parasites that feed on nestling blood. In contrast to initial predictions, P. azurea larvae were less prevalent at lower elevations. In Mediterranean environments, the colder and damper climate of medium and high elevations might favour this parasite. Alternatively, greater anthropogenic perturbation in lowland environments may have a negative impact on the parasite. The findings also show that the two host species had similar parasite loads. As coal tits are half the size of great tits, this suggests that the coal tits were more severely parasitised. In conclusion, the generalised assumption that parasite abundance decreases with elevation does not hold true for the present case and elevational parasite patterns probably depend on specific host–parasite systems and climatic conditions in the mountains.