Host-Parasite Coevolution in Cuckoos

My second current research interest is avian host-parasite co-evolution. This project allows me to work experimentally which is quite difficult in birds of prey. In this context, I am involved with fieldwork in South Africa to look at the adaptations and counter-adaptations in one particular system (the jacobin cuckoo (Clamator jacobinus) and its cape bulbul (Pycnonotus capensis) host). Through a combination of experiments and modelling, I try to explain specific features of this host-parasite system and use its relative simplicity to test general hypotheses, such as the evolutionary lag and the evolutionary equilibrium hypotheses.

This particular system offers absolutely unique possibilities to study experimentally adaptations and counter-adaptations. The cuckoo lays an egg that is completely different from the host egg and still no egg-rejection is detectable, meaning that the host has not had enough evolutionary time to evolve egg-rejection or that egg acceptance is an optimal strategy. Another striking feature is that host and parasite chicks do not produce loud begging calls and so one means for the parasite to induce the host to preferentially feed it is not available. The jacobin cuckoo chick also shows pre-incubation development which gives the chick a head-start, but the significance of this adaptation remains untested. I am in the process of obtaining a fitness data set of the host species population which would enable me to quantify the strength of selection pressure operating on the host to evolve defences against brood parasitism. This approach has almost never been feasible due to the difficulties obtaining fitness data for host species. Since the jacobin cuckoo does not always reduce host breeding success to zero if it parasitizes (host and cuckoo chick are sometimes raised together), this host-parasite system is at an intermediate stage of the co-evolutionary arms race and might provide important insights into avian brood parasitism and the co-evolutionary process in general.

I have also combined experiments and modelling to test whether the lack of counter-adaptations by the host is best explained by the evolutionary lag (the host will be selected to evolve a defence) or evolutionary equilibrium (costs of evolving a defence are higher than the benefits of that defence) hypotheses. It seems not only that the evolutionary equilibrium hypothesis is more likely, but so far unreported from any co-evolution studies, increased parasitism selects for fewer defences in the host.