Host-parasite co-evolution

During our work on woodpecker finches, we accidentally discovered the dipterian bird parasite Philornis downsi on Santa Cruz, Galápagos (Fessl &Tebbich 2002). Adult flies lay their eggs in bird nests where their parasitic larvae hatch (Fessl et al. 2001]. The first instar larvae usually develop in the chicks’ nostrils, whereas the second and third instar larvae live in the bottom layer of the nest and suck blood from the nestlings during the night. This obligate bird parasite was probably introduced to the Galapagos Islands during the last century and is now seen as one of the biggest threats to the Galapagos avifauna and involved in the rapid decline of several Darwin’s finch species.

Several female flies lay their eggs in one nest and the fly larvae need approximately 12 days until they pupate. However, due to the very high parasite load, chicks very often die within the first days and larvae have not enough time to develop to the final larval stage. During the last field season, we found indications that Philornis is under selection to speed up its life cycle: Instead of laying their eggs in the nostrils of the chicks, the flies have started to lay their eggs already into nests during the incubation phase. Stable isotope analysis indicates that the larvae now feed from the blood of the incubating females (Tebbich et al. in prep). This early infestation leads to high infestation rates in the early chick phase and results in higher mortality (Cimadom et al. 2014).

The parasitism by Philornis and the introduction of several pathogens that are transmitted by mosquitoes and affect Darwin’s finches could also be the reason for another behavioural innovation in Darwin’s finches. In 2012, we observed for the first time that several Darwin’s finch species chew the leaves of an endemic aromatic tree (Psidium galapageium) and rub the chewed leaves into their feathers. Our experiments showed that these leaves have repellent properties against mosquitoes and inhibit growth oh Philornis larvae. 


Researcher: Sabine Tebbich