Macroevolutionary outcomes of coevolution between avian brood parasites and their hosts

Abstract

Almost one hundred bird species in the world are known to be obligate interspecific brood parasites. These lay their eggs in the nests of other species, their hosts, which take care of a usually larger parasitic chick. Brood parasitism constitutes one of the best examples of coevolution in the animal kingdom. This strategy is usually costly to the host, and has led to the evolution of a suite of adaptations in hosts, in order to defend themselves against parasitism, and in parasites, in order to effectively parasitize their hosts. In this thesis I explore the effects of brood parasitism on macro-evolutionary patterns in both hosts and parasites.

In the first six chapters of my thesis I explore how defences evolve in hosts. First I present a literature review about the evolution of egg acceptance and tolerance mechanisms in hosts of brood parasites, in which I discuss how other co-evolutionary interactions, such as those between plants and herbivores, may be informative for understanding brood parasitic systems. In the second chapter I perform a large-scale comparative analysis on the evolution of clutch size as a tolerance mechanism in hosts. This chapter also incorporates a mathematical model and a field experiment on the Horsfield’s bronze-cuckoo Chalcites basalis. In the third chapter I investigate why one type of defence, egg rejection, evolves in some host species and not in others. In the fourth chapter I present a comparative analysis which tests the idea that the benefits of group defence against brood parasites has led to the evolution of cooperative breeding in hosts. For the fifth and sixth chapters, I describe field experiments to test the evolution of defences in the yellow-rumped thornbill (Acanthiza chrysorrhoa), the main host of the shining bronze-cuckoo (Chalcites lucidus) in Australia. My main aim is to understand which types of defences have evolved in this major host. I also perform field experiments to understand which factors constrain the evolution of defences in this species.

In the second part of my thesis I study how brood parasitism can be associated with the evolution of diversity in both hosts and parasites, especially in traits that are likely to be under selective pressures, such as the egg phenotype. In chapter 7 I study how egg phenotype has evolved to be more diverse within and among species that are hosts of brood parasites. In chapter 8 I explore whether a brood parasitic breeding strategy promotes the generation of new species and phenotypic diversity. Specifically, I test whether brood parasitic lineages have faster rates of speciation and phenotypic evolution.

Finally, in chapter 9, I discuss how together, these chapters offer a broad evolutionary landscape that demonstrate the diverse impacts of brood parasitism as a co-evolutionary interaction. I provide evidence that brood parasitism, besides driving the evolution of defenses, is linked to trait diversity, and may be an important force behind the evolution of clutch size, cooperative breeding, egg pattern, egg size and plumage diversity.