Quite often, very interesting questions are not amenable to experimental manipulation. The comparative approach, where different taxa are compared, has proved to be a successful alternative. Over the last 20 years, radical new methods have become available which have increased analytical power. They take a phylogeny explicitly into account and hence deal effectively with the problem of phylogenetic inertia (closely related species have similar trait values due to common descent). In addition, these new methods allow testing of the temporal ordering of evolutionary events, so evolutionary pathways can be established. This increases the explanatory power of the comparative approach immensely.
I have used comparative approaches to explain the evolution of brood parasitism in cuckoos. The results seem to indicate that the evolution of brood parasitism is a consequence of changes in ecology, rather than the cause of these changes. These results were developed further together with N. Davies from the University of Cambridge and M. Sorenson from the University of Boston. We looked at the diversity of breeding strategies and life histories across the whole family of cuckoos and study life-history evolution and speciation. Here, results indicated that the evolution of brood parasitism promotes speciation as a result of increasing adaptation to a particular host. In addition, I have used comparative approaches to study the evolution of life history traits in birds of prey and owls, sexual size dimorphism and speciation.
Recently, we have used comparative approaches to test whether the evolution of sexual size dimorphism in seals has evolved prior to the evolution of harems or whether harems have selected for progressively larger males.